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

tools/dom/templates/html/impl/impl_Element.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;
 
 class _ChildrenElementList extends ListBase<Element>
     implements NodeListWrapper {
   // Raw Element.
   final Element _element;
@@ skipping 1350 matching lines @@
   @DomName('Element.scrollHeight')
   @DocsEditable()
   int get scrollHeight => JS('num', '#.scrollHeight', this).round();
 
   @DomName('Element.scrollLeft')
   @DocsEditable()
   int get scrollLeft => JS('num', '#.scrollLeft', this).round();
 
   @DomName('Element.scrollLeft')
   @DocsEditable()
-  void set scrollLeft(int value) => JS("void", "#.scrollLeft = #", this, value.round());
+  void set scrollLeft(int value) {
+    JS("void", "#.scrollLeft = #", this, value.round());
+  }
 
   @DomName('Element.scrollTop')
   @DocsEditable()
   int get scrollTop => JS('num', '#.scrollTop', this).round();
 
   @DomName('Element.scrollTop')
   @DocsEditable()
-  void set scrollTop(int value) => JS("void", "#.scrollTop = #", this, value.round());
+  void set scrollTop(int value) {
+    JS("void", "#.scrollTop = #", this, value.round());
+  }
 
   @DomName('Element.scrollWidth')
   @DocsEditable()
   int get scrollWidth => JS('num', '#.scrollWidth', this).round();
 
 $else
   @DomName('Element.offsetHeight')
   @DocsEditable()
   int get offsetHeight => _blink.BlinkElement.$offsetHeight_Getter(this).round();
 
@@ skipping 88 matching lines @@
   const ScrollAlignment._internal(this._value);
   toString() => 'ScrollAlignment.$_value';
 
   /// Attempt to align the element to the top of the scrollable area.
   static const TOP = const ScrollAlignment._internal('TOP');
   /// Attempt to center the element in the scrollable area.
   static const CENTER = const ScrollAlignment._internal('CENTER');
   /// Attempt to align the element to the bottom of the scrollable area.
   static const BOTTOM = const ScrollAlignment._internal('BOTTOM');
 }