Remove "unnecessary" imports. (#25736)

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 https://github.com/dart-lang/sdk/issues/44569 for more information.

lib/web_ui/lib/src/engine/html/path_to_svg_clip.dart

@@ -5,8 +5,6 @@
 import 'package:ui/src/engine.dart';
 import 'package:ui/ui.dart' as ui;
 
-import '../browser_detection.dart';
-
 /// Counter used for generating clip path id inside an svg <defs> tag.
 int _clipIdCounter = 0;
 

lib/web_ui/lib/src/engine/html/shaders/normalized_gradient.dart

@@ -7,7 +7,6 @@ import 'dart:typed_data';
 import 'package:ui/src/engine.dart';
 
 import 'package:ui/ui.dart' as ui;
-import 'shader_builder.dart';
 
 /// Converts colors and stops to typed array of bias, scale and threshold to use
 /// in shaders.

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;
 

lib/web_ui/test/engine/surface/scene_builder_test.dart

@@ -12,7 +12,6 @@ import 'dart:js_util' as js_util;
 import 'package:test/bootstrap/browser.dart';
 import 'package:test/test.dart';
 import 'package:ui/src/engine.dart';
-import 'package:ui/src/engine/html_image_codec.dart';
 import 'package:ui/ui.dart' hide window;
 
 

lib/web_ui/test/engine/surface/shaders/shader_builder_test.dart

@@ -6,7 +6,6 @@
 import 'package:test/bootstrap/browser.dart';
 import 'package:test/test.dart';
 import 'package:ui/src/engine.dart';
-import 'package:ui/src/engine/html/shaders/shader_builder.dart';
 import 'package:ui/ui.dart' hide window;
 
 void main() {