Add the build option when using dart2 preview

[rkowase]

Add the description of the build apk option when using dart2 preview.
If you build without this option, NoSuchMethodError will occur.

[goderbauer]

@aam Is this a bug or intentional?

[aam]

If you build without this option, NoSuchMethodError will occur.

This sounds like a bug. --preview-dart-2 is defaulted to true, so one doesn't have to specify it explicitly.

[aam]

FWIW, flutter build apk works fine for me on master:

$FH/flutter/examples/flutter_gallery$ ../../bin/flutter build apk
Running "flutter packages get" in flutter_gallery...         1.1s
Initializing gradle...                                       0.9s
Resolving dependencies...                                    8.1s
Running 'gradlew assembleRelease'...                             
Skipping AOT snapshot build. Fingerprint match.
Built build/app/outputs/apk/release/app-release.apk (15.4MB).
$FH/flutter/examples/flutter_gallery$  ../../bin/flutter doctor -v
[✓] Flutter (Channel master, v0.2.5-pre.86, on Linux, locale en_US.UTF-8)
    • Flutter version 0.2.5-pre.86 at $FH/flutter
    • Framework revision 0c89920069 (16 hours ago), 2018-04-01 17:35:05 -0700
    • Engine revision c903c217a1
    • Dart version 2.0.0-dev.43.0.flutter-52afcba357

[mit-mit]

@rkowase can you elaborate on what issue you are seeing?

[Hixie]

@rkowase Let's start with filing a bug on this, and we'll move from there. I'm going to close this PR for now since it's not clear what the issue is.