asset name in AndroidStudio should suggest auto-completions from assets configured in pubspec.yaml

@d-silveira commented on [Tue Aug 14 2018](https://github.com/flutter/flutter/issues/20571)

1. When for instance creating an ``Image.asset()`` in AndroidStudio the assets name should suggest auto-completions from the assets already configured in pubspec.yaml.
2. On a second revision a preview of the asset should appear both inline and on the documentation tooltip.
3. On a third revision, it would be nice to have an option to add an asset from file (for when you forgot to add the asset you wanted to pubspec.yaml), which would automatically add it to a default assets folder, and also add it to pubspec.yaml, run a packages get, and finally auto-complete it.

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@zoechi commented on [Wed Aug 22 2018](https://github.com/flutter/flutter/issues/20571#issuecomment-415102989)

A first step could be a Dart file generated with string constants for each path. This way assets could be selected like icons with `Icons.sear...`.
I thought I saw a PR about that already a while ago but wasn't able to find it just now.

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@zoechi commented on [Wed Aug 22 2018](https://github.com/flutter/flutter/issues/20571#issuecomment-415103157)

Should this be in flutter/flutter-intellij ?