* 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>.
0556e14f18