Fix for getContextAttributes which wraps bare dictionary into a Dart type.

tests/html/webgl_1_test.dart

 // Copyright (c) 2013, 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 web_gl_test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 import 'dart:typed_data';
 import 'dart:web_gl';
@@ skipping 70 matching lines @@
         canvas = new CanvasElement();
         document.body.children.add(canvas);
         var context2 = canvas.getContext('2d');
         context.texSubImage2DData(1, 1, 1, 1, 1, 10,
             context2.getImageData(10, 10, 10, 10));
 
         context.texSubImage2DImage(1, 1, 1, 1, 1, 10, new ImageElement());
         context.texSubImage2DCanvas(1, 1, 1, 1, 1, 10, new CanvasElement());
         context.texSubImage2DVideo(1, 1, 1, 1, 1, 10, new VideoElement());
       });
+
+      test('getContextAttributes', () {
+        var canvas = new CanvasElement();
+        var context = canvas.getContext3d();
+        var attributes = context.getContextAttributes();
+
+        expect(attributes, isNotNull);
+        expect(attributes, new isInstanceOf<gl.ContextAttributes>());
+
+        expect(attributes.alpha, isBoolean);
+        expect(attributes.antialias, isBoolean);
+        expect(attributes.depth, isBoolean);
+        expect(attributes.premultipliedAlpha, isBoolean);
+        expect(attributes.preserveDrawingBuffer, isBoolean);
+        expect(attributes.stencil, isBoolean);
+      });
     }
   });
 }
 
+Matcher isBoolean = anyOf(isTrue, isFalse);