Repository: flutter
Issue: Verbose Linux build output contains double prefixes
Labels: tool,a: quality,P3,team-tool,triaged-tool

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## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Run `flutter create --plaforms linux .`
2. Run `flutter build linux -v`
3. Observe prefixes in square brackets (`[    XXms]`) in the build output

**Expected results:** <!-- what did you want to see? -->
Consistent prefixes

**Actual results:** <!-- what did you see? -->
The build step that calls `tool_backend.sh` -> `dart tool_backend.dart` has two prefixes. For example:

```
[        ] [   +5 ms] unpack_linux: Complete
```

<details><summary>Verbose build & Flutter doctor output</summary>

```
$ flutter build linux -v
[ +139 ms] executing: [/home/jpnurmi/Flutter/] git -c log.showSignature=false log -n 1 --pretty=format:%H
[  +60 ms] Exit code 0 from: git -c log.showSignature=false log -n 1 --pretty=format:%H
[        ] cf6c33e58a05b6bf45d8636cb27342709043e745
[   +1 ms] executing: [/home/jpnurmi/Flutter/] git tag --points-at cf6c33e58a05b6bf45d8636cb27342709043e745
[  +18 ms] Exit code 0 from: git tag --points-at cf6c33e58a05b6bf45d8636cb27342709043e745
[   +2 ms] executing: [/home/jpnurmi/Flutter/] git describe --match *.*.* --long --tags cf6c33e58a05b6bf45d8636cb27342709043e745
[  +34 ms] Exit code 0 from: git describe --match *.*.* --long --tags cf6c33e58a05b6bf45d8636cb27342709043e745
[        ] 1.24.0-7.0.pre-343-gcf6c33e58a
[  +98 ms] executing: [/home/jpnurmi/Flutter/] git rev-parse --abbrev-ref --symbolic @{u}
[   +5 ms] Exit code 0 from: git rev-parse --abbrev-ref --symbolic @{u}
[        ] origin/master
[        ] executing: [/home/jpnurmi/Flutter/] git ls-remote --get-url origin
[   +4 ms] Exit code 0 from: git ls-remote --get-url origin
[        ] git@github.com:flutter/flutter.git
[  +64 ms] executing: [/home/jpnurmi/Flutter/] git rev-parse --abbrev-ref HEAD
[   +4 ms] Exit code 0 from: git rev-parse --abbrev-ref HEAD
[        ] master
[  +71 ms] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterWebSdk' is not required, skipping update.
[   +3 ms] Artifact Instance of 'WindowsEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[  +90 ms] Artifact Instance of 'MaterialFonts' is not required, skipping update.
[        ] Artifact Instance of 'GradleWrapper' is not required, skipping update.
[        ] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterWebSdk' is not required, skipping update.
[        ] Artifact Instance of 'FlutterSdk' is not required, skipping update.
[        ] Artifact Instance of 'WindowsEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[   +2 ms] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FontSubsetArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'PubDependencies' is not required, skipping update.
[ +104 ms] Skipping pub get: version match.
[ +214 ms] Building Linux application...
[  +16 ms] executing: [build/linux/release/] cmake -G Ninja -DCMAKE_BUILD_TYPE=Release /home/jpnurmi/Temp/verbose_linux_build/linux
[ +335 ms] -- The CXX compiler identification is Clang 9.0.1
[   +2 ms] -- Detecting CXX compiler ABI info
[ +304 ms] -- Detecting CXX compiler ABI info - done
[  +29 ms] -- Check for working CXX compiler: /usr/bin/clang++ - skipped
[        ] -- Detecting CXX compile features
[   +1 ms] -- Detecting CXX compile features - done
[  +11 ms] -- Found PkgConfig: /usr/bin/pkg-config (found version "0.29.2") 
[   +1 ms] -- Checking for module 'gtk+-3.0'
[  +34 ms] --   Found gtk+-3.0, version 3.24.23
[ +181 ms] -- Checking for module 'glib-2.0'
[  +24 ms] --   Found glib-2.0, version 2.66.2
[  +63 ms] -- Checking for module 'gio-2.0'
[  +24 ms] --   Found gio-2.0, version 2.66.2
[  +79 ms] -- Checking for module 'blkid'
[  +23 ms] --   Found blkid, version 2.36.0
[  +63 ms] -- Checking for module 'liblzma'
[  +25 ms] --   Found liblzma, version 5.2.4
[  +79 ms] -- Configuring done
[  +15 ms] -- Generating done
[  +18 ms] -- Build files have been written to: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release
[  +26 ms] executing: ninja -C build/linux/release install
[  +12 ms] ninja: Entering directory `build/linux/release'
[+38462 ms] [1/6] Generating /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/libflutter_linux_gtk.so,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_basic_message_channel.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_binary_codec.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_binary_messenger.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_dart_project.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_engine.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_json_message_codec.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_json_method_codec.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_message_codec.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_method_call.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_method_channel.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_method_codec.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_method_response.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_plugin_registrar.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_plugin_registry.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_standard_message_codec.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_standard_method_codec.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_string_codec.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_value.h,
/home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/fl_view.h, /home/jpnurmi/Temp/verbose_linux_build/linux/flutter/ephemeral/flutter_linux/flutter_linux.h, _phony_
[  +21 ms] [ +188 ms] executing: [/home/jpnurmi/Flutter/] git -c log.showSignature=false log -n 1 --pretty=format:%H
[        ] [  +59 ms] Exit code 0 from: git -c log.showSignature=false log -n 1 --pretty=format:%H
[        ] [        ] cf6c33e58a05b6bf45d8636cb27342709043e745
[        ] [   +1 ms] executing: [/home/jpnurmi/Flutter/] git tag --points-at cf6c33e58a05b6bf45d8636cb27342709043e745
[        ] [  +18 ms] Exit code 0 from: git tag --points-at cf6c33e58a05b6bf45d8636cb27342709043e745
[        ] [   +2 ms] executing: [/home/jpnurmi/Flutter/] git describe --match *.*.* --long --tags cf6c33e58a05b6bf45d8636cb27342709043e745
[        ] [  +37 ms] Exit code 0 from: git describe --match *.*.* --long --tags cf6c33e58a05b6bf45d8636cb27342709043e745
[        ] [        ] 1.24.0-7.0.pre-343-gcf6c33e58a
[        ] [ +110 ms] executing: [/home/jpnurmi/Flutter/] git rev-parse --abbrev-ref --symbolic @{u}
[        ] [   +4 ms] Exit code 0 from: git rev-parse --abbrev-ref --symbolic @{u}
[        ] [        ] origin/master
[        ] [        ] executing: [/home/jpnurmi/Flutter/] git ls-remote --get-url origin
[        ] [   +4 ms] Exit code 0 from: git ls-remote --get-url origin
[        ] [        ] git@github.com:flutter/flutter.git
[        ] [  +67 ms] executing: [/home/jpnurmi/Flutter/] git rev-parse --abbrev-ref HEAD
[        ] [   +5 ms] Exit code 0 from: git rev-parse --abbrev-ref HEAD
[        ] [        ] master
[        ] [  +68 ms] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'FlutterWebSdk' is not required, skipping update.
[        ] [   +3 ms] Artifact Instance of 'WindowsEngineArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[        ] [  +83 ms] Artifact Instance of 'MaterialFonts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'GradleWrapper' is not required, skipping update.
[        ] [        ] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'FlutterWebSdk' is not required, skipping update.
[        ] [  +13 ms] Artifact Instance of 'FlutterSdk' is not required, skipping update.
[        ] [        ] Artifact Instance of 'WindowsEngineArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[        ] [        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'IosUsbArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'FontSubsetArtifacts' is not required, skipping update.
[        ] [        ] Artifact Instance of 'PubDependencies' is not required, skipping update.
[        ] [ +156 ms] Initializing file store
[        ] [  +38 ms] Skipping target: gen_localizations
[        ] [  +10 ms] unpack_linux: Starting due to {}
[        ] [ +104 ms] kernel_snapshot: Starting due to {}
[        ] [   +5 ms] unpack_linux: Complete
[        ] [+1695 ms] /home/jpnurmi/Flutter/bin/cache/dart-sdk/bin/dart --disable-dart-dev /home/jpnurmi/Flutter/bin/cache/artifacts/engine/linux-x64/frontend_server.dart.snapshot --sdk-root
/home/jpnurmi/Flutter/bin/cache/artifacts/engine/common/flutter_patched_sdk_product/ --target=flutter --no-print-incremental-dependencies -Ddart.developer.causal_async_stacks=false -Ddart.vm.profile=false
-Ddart.vm.product=true --aot --tfa --packages /home/jpnurmi/Temp/verbose_linux_build/.dart_tool/package_config.json --output-dill
/home/jpnurmi/Temp/verbose_linux_build/.dart_tool/flutter_build/4cf4fecc7e2dfc5b6e635efc3e96ba3d/app.dill --depfile
/home/jpnurmi/Temp/verbose_linux_build/.dart_tool/flutter_build/4cf4fecc7e2dfc5b6e635efc3e96ba3d/kernel_snapshot.d package:verbose_linux_build/main.dart
[   +3 ms] [+22586 ms] kernel_snapshot: Complete
[        ] [ +635 ms] aot_elf_release: Starting due to {InvalidatedReason.inputChanged}
[        ] [  +11 ms] executing: /home/jpnurmi/Flutter/bin/cache/artifacts/engine/linux-x64-release/gen_snapshot --deterministic --snapshot_kind=app-aot-elf
--elf=/home/jpnurmi/Temp/verbose_linux_build/.dart_tool/flutter_build/4cf4fecc7e2dfc5b6e635efc3e96ba3d/app.so --strip --no-causal-async-stacks --lazy-async-stacks
/home/jpnurmi/Temp/verbose_linux_build/.dart_tool/flutter_build/4cf4fecc7e2dfc5b6e635efc3e96ba3d/app.dill
[   +1 ms] [+10992 ms] aot_elf_release: Complete
[        ] [  +52 ms] linux_aot_bundle: Starting due to {InvalidatedReason.inputChanged}
[        ] [  +80 ms] linux_aot_bundle: Complete
[        ] [  +53 ms] release_bundle_linux_assets: Starting due to {}
[        ] [ +200 ms] release_bundle_linux_assets: Complete
[        ] [  +64 ms] Persisting file store
[        ] [  +10 ms] Done persisting file store
[        ] [   +8 ms] build succeeded.
[        ] [  +15 ms] "flutter assemble" took 36,841ms.
[        ] [   +7 ms] Running shutdown hooks
[        ] [        ] Shutdown hooks complete
[        ] [        ] exiting with code 0
[+1302 ms] [2/6] Building CXX object CMakeFiles/verbose_linux_build.dir/main.cc.o
[  +52 ms] [3/6] Building CXX object CMakeFiles/verbose_linux_build.dir/flutter/generated_plugin_registrant.cc.o
[ +311 ms] [4/6] Building CXX object CMakeFiles/verbose_linux_build.dir/my_application.cc.o
[ +266 ms] [5/6] Linking CXX executable intermediates_do_not_run/verbose_linux_build
[        ] [5/6] Install the project...
[  +14 ms] -- Install configuration: "Release"
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/verbose_linux_build
[        ] -- Set runtime path of "/home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/verbose_linux_build" to "$ORIGIN/lib"
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/icudtl.dat
[   +1 ms] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/lib/libflutter_linux_gtk.so
[  +88 ms] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/version.json
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/NOTICES
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/FontManifest.json
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/packages
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/packages/cupertino_icons
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/packages/cupertino_icons/assets
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/packages/cupertino_icons/assets/CupertinoIcons.ttf
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/fonts
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/fonts/MaterialIcons-Regular.otf
[   +1 ms] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/data/flutter_assets/AssetManifest.json
[        ] -- Installing: /home/jpnurmi/Temp/verbose_linux_build/build/linux/release/bundle/lib/libapp.so
[  +25 ms] Building Linux application... (completed in 41.9s)
[   +1 ms] "flutter linux" took 42,402ms.
[   +8 ms] Running shutdown hooks
[        ] Shutdown hooks complete
[   +1 ms] exiting with code 0
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
$ flutter doctor -v
[✓] Flutter (Channel master, 1.24.0-8.0.pre.343, on Linux, locale en_DK.UTF-8)
    • Flutter version 1.24.0-8.0.pre.343 at /home/jpnurmi/Flutter
    • Framework revision cf6c33e58a (11 hours ago), 2020-11-21 14:04:01 -0800
    • Engine revision 23a8e027db
    • Dart version 2.12.0 (build 2.12.0-62.0.dev)

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
    • Android SDK at /home/jpnurmi/Android/sdk
    • Platform android-29, build-tools 29.0.3
    • ANDROID_SDK_ROOT = /home/jpnurmi/Android/sdk
    • Java binary at: /usr/lib/jvm/default-java/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.9.1+1-post-Debian-1)
    • All Android licenses accepted.

[✓] Chrome - develop for the web
    • Chrome at google-chrome

[✓] Linux toolchain - develop for Linux desktop
    • clang version 9.0.1-15
    • cmake version 3.18.4
    • ninja version 1.10.1
    • pkg-config version 0.29.2

[!] Android Studio (not installed)
    • Android Studio not found; download from https://developer.android.com/studio/index.html
      (or visit https://flutter.dev/docs/get-started/install/linux#android-setup for detailed instructions).

[✓] VS Code (version 1.51.1)
    • VS Code at /usr/share/code
    • Flutter extension version 3.16.0

[✓] Connected device (3 available)
    • Linux (desktop)  • linux      • linux-x64      • Linux
    • Web Server (web) • web-server • web-javascript • Flutter Tools
    • Chrome (web)     • chrome     • web-javascript • Google Chrome 87.0.4280.66

! Doctor found issues in 1 category.
```

</details>