Revert "Making all DOM types abstract and removing superfluous Dartium constructors."

tools/dom/templates/html/impl/impl_HTMLCanvasElement.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.
 
 part of $LIBRARYNAME;
 
-$(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS implements CanvasImageSource$IMPLEMENTS$NATIVESPEC {
+$(ANNOTATIONS)class $CLASSNAME$EXTENDS implements CanvasImageSource$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
   /** An API for drawing on this canvas. */
 $if DART2JS
   CanvasRenderingContext2D get context2D =>
       JS('Null|CanvasRenderingContext2D', '#.getContext(#)', this, '2d');
 $else
   CanvasRenderingContext2D get context2D => getContext('2d');
 $endif
 
   @deprecated
@@ skipping 13 matching lines @@
       'premultipliedAlpha': premultipliedAlpha,
       'preserveDrawingBuffer': preserveDrawingBuffer,
     };
     var context = getContext('webgl', options);
     if (context == null) {
       context = getContext('experimental-webgl', options);
     }
     return context;
   }
 }