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

tests/html/canvasrenderingcontext2d_test.dart

 // 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.
 
 library canvas_rendering_context_2d_test; import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 import 'dart:math';
 
 // Some rounding errors in the browsers.
@@ skipping 225 matching lines @@
       // TODO(antonm): in Dartium ArgumentError should be thrown too.
       expect(() => context.putImageData(expectedData, 0, 0, 1),
         throws);
       expect(() => context.putImageData(expectedData, 0, 0, 1, 1),
         throws);
       expect(() => context.putImageData(expectedData, 0, 0, 1, 1, 5),
         throws);
     });
   });
 
+  group('linedash', () {
+    setUp(setupFunc);
+    tearDown(tearDownFunc);
+
+    test('setLineDash', () {
+      expect(context.getLineDash(), equals([]));
+      var old = context.getLineDash();
+      context.setLineDash(old);
+    });
+  });
+
   group('arc', () {
     setUp(setupFunc);
     tearDown(tearDownFunc);
 
     test('default arc should be clockwise', () {
       context.beginPath();
       final r = 10;
 
       // Center of arc.
       final cx = 20;
@@ skipping 482 matching lines @@
 
       // The box does not draw after 20 pixels.
       expectPixelUnfilled(x - 10, y);
       expectPixelUnfilled(x + maxWidth + 1, y);
       expectPixelUnfilled(x + maxWidth + 20, y);
       expectPixelFilled(x, y);
       expectPixelFilled(x + 10, y);
     });
   });
 }