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

tests/html/canvasrenderingcontext2d_test.dart

Index: tests/html/canvasrenderingcontext2d_test.dart
diff --git a/tests/html/canvasrenderingcontext2d_test.dart b/tests/html/canvasrenderingcontext2d_test.dart
index 3f17dc921cc39fa48b69916dbf58e1c2a0bc0d89..3d2b27e13a7b8320d7680bd83766b9e0bd145acd 100644
--- a/tests/html/canvasrenderingcontext2d_test.dart
+++ b/tests/html/canvasrenderingcontext2d_test.dart
@@ -243,6 +243,17 @@ main() {
     });
   });
 
+  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);