* master: (88 commits)
  Upgrade dartdoc to 0.16.0. (flutter#14602)
  Reduce noise in xcodebuild stdout (flutter#14586)
  Add annotations to ignore analyzer errors on pre-dev.22 SDKs (flutter#14599)
  Roll engine to 337764e (flutter#14597)
  Roll engine with rolled dart (flutter#14538)
  Upgrade packages (flutter#14588)
  Change GlobalObjectKey.toString to strip away State<StatefulWidget>. (flutter#14558)
  Disable selection of the Android ARM64 target platform based on the attached device (flutter#14581)
  Add a flutter run option that can override the default target platform (flutter#14537)
  Fix message channel usage in few tests. (flutter#14557)
  Update documentation on how to test flutter_tools (flutter#14567)
  Fix text_style_text.dart to be Dart 2 compliant. (flutter#14559)
  Hide the NDK warning that should never happen with a regexp (flutter#14503)
  Fix iOS build which broke after the change to fuse --strong option into --preview-dart-2.
  Mark run_machine_concurrent_hot_reload as flaky (flutter#14563)
  Fuse --strong into --preview-dart-2 option.
  Partially fix Dart 2 issues in animated_icons.test. (flutter#14531)
  Strong mode fixes in tests (flutter#14520)
  Fix typo.
  Fix issues.
  ...

* Use explicit types for onClick handler in diagnostics_test.

This test formats it to string and expects exact match.

Before this change onClick had type () => dynamic in Dart 1 and () => Null in
Dart 2.

* Fix strong mode issues in packages/flutter/test/material/dropdown_test.dart

Pass explicit type argument to renderObjectList(...) in expressions like

     List<RenderBox> l = tester.renderObjectList(...).toList();

Dart 2 mode inference is not strong enough to infer RenderBox as a type argument
fto renderObjectList and passes RenderObject instead, which later leads to
runtime check failures (because List<RenderObject> is not List<RenderBox>).

* Fix strong mode issues in packages/flutter/test/material/theme_test.dart.

Pass type argument explicity to widgetList. Dart 2 type inference can't infer
it by itself.

* Fix strong mode issue packages/flutter/test/widgets/unique_widget_test.dart

Pass correct type argument to GlobalKey.

* Fix type annotation in packages/flutter/test/material/app_test.dart.

pushNamed returns Future<Object> not Future<String>.