More precise native type for context2D

tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate

Index: tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate b/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
index 82bf5a766032ca6d9753fc0deef37a4552ec3a57..ef01381ece5b7505308c93fb7ff772ab91e62765 100644
--- a/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
@@ -7,7 +7,12 @@ part of $LIBRARYNAME;
 $(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
   CanvasRenderingContext2D get context2d => this.context2D;