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

tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate

Index: tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate b/tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate
index 1068fdcaae3767a9fc0e7f085d71829767088e85..4e92d9a7d705c1d94aa8ee2cf422e9a9dc0513b3 100644
--- a/tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate
+++ b/tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate
@@ -270,9 +270,11 @@ $if DART2JS
   @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.