[multiple] Avoid new unreachable_switch_default warning. (flutter/engine#54996)

The Dart analyzer will soon be changed so that if the `default` clause
of a `switch` statement is determined to be unreachable by the
exhaustiveness checker, a new warning of type
`unreachable_switch_default` will be issued. This parallels the behavior
of the existing `unreachable_switch_case` warning, which is issued
whenever a `case` clause of a `switch` statement is determined to be
unreachable.

Before adding the new warning to the analyzer, code in the engine needs
to first be updated to eliminate these unreachable `default` clauses, so
that the warning won't cause builds to break.

For more information, see
https://github.com/dart-lang/sdk/issues/54575.

engine/src/flutter/lib/web_ui/lib/src/engine/canvaskit/color_filter.dart

@@ -266,7 +266,5 @@ CkColorFilter? createCkColorFilter(EngineColorFilter colorFilter) {
         return const CkLinearToSrgbGammaColorFilter();
       case ColorFilterType.srgbToLinearGamma:
         return const CkSrgbToLinearGammaColorFilter();
-      default:
-        throw StateError('Unknown mode $colorFilter.type for ColorFilter.');
     }
 }

engine/src/flutter/lib/web_ui/lib/src/engine/canvaskit/embedded_views.dart

@@ -844,8 +844,6 @@ class Mutator {
         return matrix == typedOther.matrix;
       case MutatorType.opacity:
         return alpha == typedOther.alpha;
-      default:
-        return false;
     }
   }
 

engine/src/flutter/lib/web_ui/lib/src/engine/html/bitmap_canvas.dart

@@ -1337,7 +1337,6 @@ String? stringForStrokeCap(ui.StrokeCap? strokeCap) {
     case ui.StrokeCap.round:
       return 'round';
     case ui.StrokeCap.square:
-    default:
       return 'square';
   }
 }
@@ -1349,7 +1348,6 @@ String stringForStrokeJoin(ui.StrokeJoin strokeJoin) {
     case ui.StrokeJoin.bevel:
       return 'bevel';
     case ui.StrokeJoin.miter:
-    default:
       return 'miter';
   }
 }

engine/src/flutter/lib/web_ui/lib/src/engine/html/shaders/shader.dart

@@ -914,7 +914,5 @@ EngineHtmlColorFilter? createHtmlColorFilter(EngineColorFilter? colorFilter) {
         throw UnimplementedError('ColorFilter.linearToSrgbGamma not implemented for HTML renderer');
       case ColorFilterType.srgbToLinearGamma:
         throw UnimplementedError('ColorFilter.srgbToLinearGamma not implemented for HTML renderer.');
-      default:
-        throw StateError('Unknown mode $colorFilter.type for ColorFilter.');
     }
 }

engine/src/flutter/lib/web_ui/lib/src/engine/text/paragraph.dart

@@ -1118,8 +1118,6 @@ String? _decorationStyleToCssString(ui.TextDecorationStyle decorationStyle) {
       return 'solid';
     case ui.TextDecorationStyle.wavy:
       return 'wavy';
-    default:
-      return null;
   }
 }
 

engine/src/flutter/lib/web_ui/lib/src/engine/text_editing/text_capitalization.dart

@@ -75,7 +75,6 @@ class TextCapitalizationConfig {
       case TextCapitalization.sentences:
         autocapitalize = 'sentences';
       case TextCapitalization.none:
-      default:
         autocapitalize = 'off';
     }
     if (domInstanceOfString(domElement, 'HTMLInputElement')) {