Fix all dart:html strong mode errors except for spurious FINAL_NOT_INITIALIZED_CONSTRUCTOR warnings.

tool/input_sdk/lib/html/html_common/conversions.dart

Index: tool/input_sdk/lib/html/html_common/conversions.dart
diff --git a/tool/input_sdk/lib/html/html_common/conversions.dart b/tool/input_sdk/lib/html/html_common/conversions.dart
index 0dca4d3f0814a4636009c64959af75f194506212..de0d423258d971447938882f573cc9e49e86fd09 100644
--- a/tool/input_sdk/lib/html/html_common/conversions.dart
+++ b/tool/input_sdk/lib/html/html_common/conversions.dart
@@ -266,7 +266,7 @@ abstract class _AcceptStructuredClone {
 // Conversions for ContextAttributes.
 //
 // On Firefox, the returned ContextAttributes is a plain object.
-class _TypedContextAttributes implements gl.ContextAttributes {
+class ContextAttributes {
   bool alpha;
   bool antialias;
   bool depth;
@@ -275,21 +275,16 @@ class _TypedContextAttributes implements gl.ContextAttributes {
   bool stencil;
   bool failIfMajorPerformanceCaveat;
 
-  _TypedContextAttributes(this.alpha, this.antialias, this.depth,
+  ContextAttributes(this.alpha, this.antialias, this.depth,
       this.failIfMajorPerformanceCaveat, this.premultipliedAlpha,
       this.preserveDrawingBuffer, this.stencil);
 }
 
-gl.ContextAttributes convertNativeToDart_ContextAttributes(
-    nativeContextAttributes) {
-  if (nativeContextAttributes is gl.ContextAttributes) {
-    return nativeContextAttributes;
-  }
-
+convertNativeToDart_ContextAttributes(nativeContextAttributes) {
   // On Firefox the above test fails because ContextAttributes is a plain
   // object so we create a _TypedContextAttributes.
 
-  return new _TypedContextAttributes(
+  return new ContextAttributes(
       JS('var', '#.alpha', nativeContextAttributes),
       JS('var', '#.antialias', nativeContextAttributes),
       JS('var', '#.depth', nativeContextAttributes),