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

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 6615ea2212b7b0045a2339b61838f994c7a47169..2c573d3fb436a072eac1b399e69ce3be65c97300 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -1745,7 +1745,7 @@ class CanvasRenderingContext2D extends CanvasRenderingContext {
 
   @DomName('CanvasRenderingContext2D.getLineDash')
   @DocsEditable()
-  List<num> getLineDash() native "CanvasRenderingContext2D_getLineDash_Callback";
+  List<num> _getLineDash() native "CanvasRenderingContext2D_getLineDash_Callback";
 
   bool isPointInPath(num x, num y, [String winding]) {
     if (winding != null) {
@@ -2082,6 +2082,15 @@ class CanvasRenderingContext2D extends CanvasRenderingContext {
         destX, destY, destWidth, destHeight);
   }
 
+  @DomName('CanvasRenderingContext2D.getLineDash')
+  List<num> getLineDash() {
+    var result = _getLineDash();
+    if (result == null) {
+      result = [];
+      setLineDash(result);
+    }
+    return result;
+  }
   // TODO(amouravski): Add Dartium native methods for drawImage once we figure
   // out how to not break native bindings.
 }