Repository: flutter
Issue: Asset paths are inconsistent 
Labels: a: assets,has reproducible steps,P2,found in release: 2.2,found in release: 2.4,team-tool,triaged-tool

## Steps to Reproduce

1. Run `flutter create asset_example`.
2. Update the files as follows: Extract the attached file:
[asset_example.zip](https://github.com/flutter/flutter/files/6631387/asset_example.zip)
3. This file adds an asset called my_asset.txt and some code to try and read it in lib/main.dart
4. Run the build on Windows with "flutter run" and select the target Windows (desktop)
5. Click on the FAB and look at the console output

**Expected results:**

The code called by clicking on the FAB, in lib/main.dart:_readAssest(), should print:

```console
flutter: *** Test #1: Calling loadString() without path
flutter: *** Success (Hello world)!
flutter: ***
flutter: *** Test #2: Calling loadString() with path
flutter: *** Success (Hello world)!
```
**Actual results:**
```console
flutter: *** Test #1: Calling loadString() without path
flutter: *** Failed, exception thrown
flutter: ***
flutter: *** Test #2: Calling loadString() with path
flutter: *** Success (Hello world)!
```
This only happens on Windows.  On MacOS, Android, iOS and Web (both on Windows and MacOS) it doesn't matter whether you call loadString('my_asset.txt') or loadString('assets/my_asset.txt').  The behaviour should be consistent across all platforms.  It took quite a bit of digging to figure out why a 3rd party plugin wasn't working on Windows due to this problem!

If you now type "flutter run" and instead select Chrome (chrome) as the target, you'll see that both calls to loadString() work.

It doesn't matter whether I add the asset to the pubspec.yaml by using the full path (- assets/my_asset.txt) or only the filename (- my_asset.txt): the results are the same.  I have to use the full path in loadString() for it to work on Windows but on all other platforms, just the filename is enough.

<details>
  <summary>Logs</summary>

<!--
      Run your application with `flutter run --verbose` and attach all the
      log output below between the lines with the backticks. If there is an
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-->

```console
[Snipped excessive output]
[   +2 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/assets
[   +2 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/**assets/my_asset.txt**
[   +3 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/FontManifest.json
[   +2 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/fonts
[   +1 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/fonts/MaterialIcons-Regular.otf
[  +24 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/kernel_blob.bin
[ +488 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/NOTICES
[ +280 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/packages
[   +1 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/packages/cupertino_icons
[   +2 ms]   -- Installing: D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/packages/cupertino_icons/assets
[   +2 ms]   -- Installing:
D:/Source/Flutter/asset_example/build/windows/runner/Debug/data/flutter_assets/packages/cupertino_icons/assets/CupertinoIcons.ttf
[  +39 ms] FinalizeBuildStatus:
[   +1 ms]   Die Datei "x64\Debug\INSTALL\INSTALL.tlog\unsuccessfulbuild" wird gelöscht.
[        ]   Aktualisieren des Timestamps von "x64\Debug\INSTALL\INSTALL.tlog\INSTALL.lastbuildstate".
[        ] Die Erstellung von Projekt "D:\Source\Flutter\asset_example\build\windows\INSTALL.vcxproj" ist abgeschlossen (Standardziele).
[        ] Der Buildvorgang wurde erfolgreich ausgeführt.
[   +1 ms]     0 Warnung(en)
[        ]     0 Fehler
[        ] Verstrichene Zeit 00:00:47.41
[  +39 ms] Building Windows application... (completed in 53,5s)
[+1925 ms] Observatory URL on device: http://127.0.0.1:60977/jbP5Nn4c03M=/
[   +5 ms] Caching compiled dill
[  +66 ms] Connecting to service protocol: http://127.0.0.1:60977/jbP5Nn4c03M=/
[ +212 ms] Launching a Dart Developer Service (DDS) instance at http://127.0.0.1:0, connecting to VM service at http://127.0.0.1:60977/jbP5Nn4c03M=/.
[  +80 ms] DDS is listening at http://127.0.0.1:60980/j2QIYS1vN84=/.
[  +73 ms] Successfully connected to service protocol: http://127.0.0.1:60977/jbP5Nn4c03M=/
[  +23 ms] DevFS: Creating new filesystem on the device (null)
[  +21 ms] DevFS: Created new filesystem on the device (file:///C:/Users/Colin/AppData/Local/Temp/asset_examplecadd69ff/asset_example/)
[   +2 ms] Updating assets
[  +91 ms] Syncing files to device Windows...
[   +3 ms] <- reset
[        ] Compiling dart to kernel with 0 updated files
[   +3 ms] <- recompile package:asset_example/main.dart 76b568cb-07ad-4824-bfb4-e0a0852698a8
[        ] <- 76b568cb-07ad-4824-bfb4-e0a0852698a8
[ +249 ms] Updating files.
[   +1 ms] DevFS: Sync finished
[        ] Syncing files to device Windows... (completed in 258ms)
[        ] Synced 0.0MB.
[   +2 ms] <- accept
[   +7 ms] Connected to _flutterView/0x16d800d0ec0.
[   +4 ms] Flutter run key commands.
[   +1 ms] r Hot reload.
[   +1 ms] R Hot restart.
[        ] h Repeat this help message.
[        ] d Detach (terminate "flutter run" but leave application running).
[        ] c Clear the screen
[        ] q Quit (terminate the application on the device).
[        ]  Running with sound null safety
[        ] An Observatory debugger and profiler on Windows is available at: http://127.0.0.1:60980/j2QIYS1vN84=/
[+1596 ms] The Flutter DevTools debugger and profiler on Windows is available at:
http://127.0.0.1:9108?uri=http%3A%2F%2F127.0.0.1%3A60980%2Fj2QIYS1vN84%3D%2F
[+2089 ms] flutter: *** Test #1: Calling loadString() without path
[  +10 ms] flutter: *** Failed, exception thrown
[        ] flutter: ***
[        ] flutter: *** Test #2: Calling loadString() with path
[   +1 ms] flutter: *** Success (Hello world)!
```



```
d:\Source\Flutter\asset_example>flutter analyze
Analyzing asset_example...
No issues found! (ran in 19.3s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
d:\Source\Flutter\asset_example>flutter doctor -v
[√] Flutter (Channel stable, 2.2.1, on Microsoft Windows [Version 10.0.19041.1052], locale de-DE)
    • Flutter version 2.2.1 at d:\Coding\Flutter
    • Framework revision 02c026b03c (2 weeks ago), 2021-05-27 12:24:44 -0700
    • Engine revision 0fdb562ac8
    • Dart version 2.13.1

[√] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at d:\Coding\Android\SDK
    • Platform android-30, build-tools 30.0.2
    • Java binary at: C:\Program Files (x86)\Common Files\Oracle\Java\javapath\java.exe
    • Java version Java(TM) SE Runtime Environment (build 1.8.0_291-b10)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop for Windows (Visual Studio Community 2019 16.10.0)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community
    • Visual Studio Community 2019 version 16.10.31321.278
    • Windows 10 SDK version 10.0.19041.0

[!] 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/windows#android-setup for detailed instructions).

[√] VS Code, 64-bit edition (version 1.56.2)
    • VS Code at C:\Program Files\Microsoft VS Code
    • Flutter extension version 3.23.0

[√] Connected device (3 available)
    • Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.19041.1052]
    • Chrome (web)      • chrome  • web-javascript • Google Chrome 91.0.4472.77
    • Edge (web)        • edge    • web-javascript • Microsoft Edge 91.0.864.41

! Doctor found issues in 1 category.
```

</details>

Many thanks for your help!