Repository: flutter
Issue: Allow native code to use asset variants in flutter_assets folder
Labels: c: new feature,engine,a: assets,P2,team-engine,triaged-engine

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## Steps to Reproduce

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When I use `registrar.lookupKeyForAsset(path)`/`[registrar lookupKeyForAsset:path]` to fetch assets defined in dart side, native side just use the path to fetch the assets rather than fetch assets depend on resolution. Or am I missing something?

<!-- Finally, paste the output of running `flutter doctor -v` here. -->
`flutter doctor -v`
```
[✓] Flutter (Channel beta, v1.0.0, on Mac OS X 10.14.1 18B75, locale en-CN)
    • Flutter version 1.0.0 at /Users/yohom/Library/Flutter/flutter
    • Framework revision 5391447fae (7 days ago), 2018-11-29 19:41:26 -0800
    • Engine revision 7375a0f414
    • Dart version 2.1.0 (build 2.1.0-dev.9.4 f9ebf21297)

[✓] Android toolchain - develop for Android devices (Android SDK 28.0.3)
    • Android SDK at /Users/yohom/Library/Android/sdk
    • Android NDK at /Users/yohom/Library/Android/sdk/ndk-bundle
    • Platform android-28, build-tools 28.0.3
    • ANDROID_HOME = /Users/yohom/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1136-b06)
    • All Android licenses accepted.

[✓] iOS toolchain - develop for iOS devices (Xcode 10.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 10.1, Build version 10B61
    • ios-deploy 1.9.4
    • CocoaPods version 1.5.3

[✓] Android Studio (version 3.2)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 31.1.1
    • Dart plugin version 181.5656
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1136-b06)

[!] IntelliJ IDEA Ultimate Edition (version 2018.3)
    • IntelliJ at /Applications/IntelliJ IDEA.app
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    • Dart plugin version 183.4733
    • For information about installing plugins, see
      https://flutter.io/intellij-setup/#installing-the-plugins

[✓] VS Code (version 1.29.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 2.20.0
```