Change void typed setters in html_dart2js not to return a value.

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;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 $!MEMBERS
 
   /**
@@ skipping 252 matching lines @@
   num get lineDashOffset => JS('num',
       '#.lineDashOffset || #.webkitLineDashOffset', this, this);
 
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
   @SupportedBrowser(SupportedBrowser.IE, '11')
   @Unstable()
   @DomName('CanvasRenderingContext2D.lineDashOffset')
   // TODO(14316): Firefox has this functionality with mozDashOffset, but it
   // needs to be polyfilled.
-  void set lineDashOffset(num value) => JS('void',
-      'typeof #.lineDashOffset != "undefined" ? #.lineDashOffset = # : '
-      '#.webkitLineDashOffset = #', this, this, value, this, value);
+  void set lineDashOffset(num value) {
+    JS('void',
+       'typeof #.lineDashOffset != "undefined" ? #.lineDashOffset = # : '
+       '#.webkitLineDashOffset = #', this, this, value, this, value);
+  }
 $else
   // TODO(amouravski): Add Dartium native methods for drawImage once we figure
   // out how to not break native bindings.
 $endif
 
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
   @SupportedBrowser(SupportedBrowser.IE, '11')
   @Unstable()
   @DomName('CanvasRenderingContext2D.getLineDash')
@@ skipping 62 matching lines @@
     JS('void', '#.fill(#)', this, winding);
   }
 $endif
 
   /** Deprecated always returns 1.0 */
   @DomName('CanvasRenderingContext2D.webkitBackingStorePixelRation')
   @Experimental()
   @deprecated
   double get backingStorePixelRatio => 1.0;
 }