Remove "unnecessary" imports.

In each library where an import is removed, the library uses some elements
provided by the import, BUT there is another import which provides all of the
same elements, and at least one more which the library uses.

In this change, we remove the imports which can be simply removed in favor of
the other already present imports.

See dart-lang/sdk#44569 for more information.

lib/web_ui/lib/src/engine/pointer_binding.dart

@@ -11,8 +11,6 @@ import 'package:meta/meta.dart';
 import 'package:ui/src/engine.dart';
 import 'package:ui/ui.dart' as ui;
 
-import 'pointer_converter.dart';
-
 /// Set this flag to true to see all the fired events in the console.
 const bool _debugLogPointerEvents = false;