Fix HTMLCanvasElement.getContext it can return RenderingContext2 now which will be tree shaken with…

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index d0cb36a56787093cb4543050b3d0d79be26844b0..c07f01bdccd841eafa8d9f2d6eec7bce4e226f75 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -39,7 +39,7 @@ import 'dart:svg' show Matrix;
 import 'dart:svg' show SvgSvgElement;
 import 'dart:web_audio' as web_audio;
 import 'dart:web_gl' as gl;
-import 'dart:web_gl' show RenderingContext;
+import 'dart:web_gl' show RenderingContext, RenderingContext2;
 import 'dart:web_sql';
 import 'dart:_isolate_helper' show IsolateNatives;
 import 'dart:_foreign_helper' show JS, JS_INTERCEPTOR_CONSTANT;
@@ -2023,8 +2023,8 @@ class CanvasElement extends HtmlElement implements CanvasImageSource {
 
   @DomName('HTMLCanvasElement.getContext')
   @DocsEditable()
-  @Creates('CanvasRenderingContext2D|RenderingContext')
-  @Returns('CanvasRenderingContext2D|RenderingContext|Null')
+  @Creates('CanvasRenderingContext2D|RenderingContext|RenderingContext2')
+  @Returns('CanvasRenderingContext2D|RenderingContext|RenderingContext2|Null')
   Object getContext(String contextId, [Map attributes]) {
     if (attributes != null) {
       var attributes_1 = convertDartToNative_Dictionary(attributes);
@@ -2036,14 +2036,14 @@ class CanvasElement extends HtmlElement implements CanvasImageSource {
   @JSName('getContext')
   @DomName('HTMLCanvasElement.getContext')
   @DocsEditable()
-  @Creates('CanvasRenderingContext2D|RenderingContext')
-  @Returns('CanvasRenderingContext2D|RenderingContext|Null')
+  @Creates('CanvasRenderingContext2D|RenderingContext|RenderingContext2')
+  @Returns('CanvasRenderingContext2D|RenderingContext|RenderingContext2|Null')
   Object _getContext_1(contextId, attributes) native;
   @JSName('getContext')
   @DomName('HTMLCanvasElement.getContext')
   @DocsEditable()
-  @Creates('CanvasRenderingContext2D|RenderingContext')
-  @Returns('CanvasRenderingContext2D|RenderingContext|Null')
+  @Creates('CanvasRenderingContext2D|RenderingContext|RenderingContext2')
+  @Returns('CanvasRenderingContext2D|RenderingContext|RenderingContext2|Null')
   Object _getContext_2(contextId) native;
 
   @DomName('HTMLCanvasElement.toBlob')