Remove Webkit prefix from all members and make experimental.

tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.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 $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
 
   /**
@@ skipping 24 matching lines @@
 
   /**
    * Sets the color used for stroking shapes.
    * [h] is in degrees, 0-360.
    * [s], [l] are in percent, 0-100.
    * [a] is 0-1.
    */
   void setStrokeColorHsl(int h, num s, num l, [num a = 1]) {
     this.strokeStyle = 'hsla($h, $s%, $l%, $a)';
   }
+
+$if DART2JS
+  @DomName('CanvasRenderingContext2D.lineDashOffset')
+  num get lineDashOffset => JS('num',
+      '#.lineDashOffset || #.webkitLineDashOffset', this, this);
+
+  @DomName('CanvasRenderingContext2D.lineDashOffset')
+  void set lineDashOffset(num value) => JS('void', 
+      'typeof #.lineDashOffset != "undefined" ? #.lineDashOffset = # : '
+      '#.webkitLineDashOffset = #', this, this, value, this, value);
+$endif
 }