Make getLineDash return [] instead of null if it has not yet been set.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 9ceaa5bfb8e4c12321459bca2bf97a6d1cecae88..3a2ff2a0b2ce2b149bcf0fd812c3cdff082110c0 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -1383,9 +1383,10 @@ class CanvasRenderingContext2D extends CanvasRenderingContext native "CanvasRend
   @Creates('ImageData|=Object')
   _getImageData_1(sx, sy, sw, sh) native;
 
+  @JSName('getLineDash')
   @DomName('CanvasRenderingContext2D.getLineDash')
   @DocsEditable()
-  List<num> getLineDash() native;
+  List<num> _getLineDash() native;
 
   @DomName('CanvasRenderingContext2D.isPointInPath')
   @DocsEditable()
@@ -1747,6 +1748,9 @@ class CanvasRenderingContext2D extends CanvasRenderingContext native "CanvasRend
       'typeof #.lineDashOffset != "undefined" ? #.lineDashOffset = # : '
       '#.webkitLineDashOffset = #', this, this, value, this, value);
 
+  @DomName('CanvasRenderingContext2D.getLineDash')
+  List<num> getLineDash() => _getLineDash();
+
   /**
    * Draws text to the canvas.
    *