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

tools/dom/templates/html/dart2js/html_dart2js.darttemplate

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // DO NOT EDIT - unless you are editing documentation as per:
 // https://code.google.com/p/dart/wiki/ContributingHTMLDocumentation
 // Auto-generated dart:html library.
 
 /**
  * HTML elements and other resources for web-based applications that need to
@@ skipping 30 matching lines @@
 import "dart:convert";
 import 'dart:math';
 import 'dart:_native_typed_data';
 import 'dart:typed_data';
 // Not actually used, but imported since dart:html can generate these objects.
 import 'dart:svg' as svg;
 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:_js_helper' show
     convertDartClosureToJS, Creates, JavaScriptIndexingBehavior,
     JSName, Native, Returns, ForceInline,
     findDispatchTagForInterceptorClass, setNativeSubclassDispatchRecord,
     makeLeafDispatchRecord;
 import 'dart:_interceptors' show
     Interceptor, JSExtendableArray, JSUInt31,
     findInterceptorConstructorForType,
     findConstructorForNativeSubclassType,
@@ skipping 86 matching lines @@
 createCustomUpgrader(Type customElementClass, $this) => $this;
 
 /**
  * Emitted for any setlike IDL entry needs a callback signature.
  * Today there is only one.
  */
 @DomName('FontFaceSetForEachCallback')
 @Experimental() // untriaged
 typedef void FontFaceSetForEachCallback(
     FontFace fontFace, FontFace fontFaceAgain, FontFaceSet set);