* 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>.