Adding supported checks for getCssCanvasContext.

tests/html/document_test.dart

Index: tests/html/document_test.dart
diff --git a/tests/html/document_test.dart b/tests/html/document_test.dart
index 2f44c2f1e0d076d1378df77447836e1562e0314b..0c6ca43e8788f3779ad9c5eebf9c7ebe8a2392a8 100644
--- a/tests/html/document_test.dart
+++ b/tests/html/document_test.dart
@@ -21,12 +21,24 @@ main() {
     expect(new Element.tag('bad_name'), isUnknownElement);
   });
 
+  group('supports_cssCanvasContext', () {
+    test('supports_cssCanvasContext', () {
+      expect(HtmlDocument.supportsCssCanvasContext, true);
+    });
+  });
+
   group('getCssCanvasContext', () {
     test('getCssCanvasContext 2d', () {
-      var context = document.getCssCanvasContext('2d', 'testContext', 10, 20);
-      expect(context is CanvasRenderingContext2D, true);
-      expect(context.canvas.width, 10);
-      expect(context.canvas.height, 20);
+      var expectation = HtmlDocument.supportsCssCanvasContext ?
+        returnsNormally : throws;
+
+      expect(() {
+        var context = document.getCssCanvasContext('2d', 'testContext', 10, 20);
+        expect(context is CanvasRenderingContext2D, true);
+        expect(context.canvas.width, 10);
+        expect(context.canvas.height, 20);
+      }, expectation);
+
     });
   });
 }