TBR: fix polyfill code to work on machines that do not support webgl.

lib/runtime/dart_library.js

 // Copyright (c) 2015, 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.
 
 /* This file defines the module loader for the dart runtime.
 */
 
 var dart_library =
   typeof module != "undefined" && module.exports || {};
 
@@ skipping 121 matching lines @@
         window.MemoryInfo = window.performance.memory.constructor;
       }
       if (typeof Geolocation == "undefined") {
         navigator.geolocation.constructor;
       }
       if (typeof Animation == "undefined") {
         let d = document.createElement('div');
         window.Animation = d.animate(d).constructor;
       }
       if (typeof WebGLVertexArrayObjectOES == "undefined") {
-        window.WebGLVertexArrayObjectOES = document.createElement('canvas')
-          .getContext('webgl')
-          .getExtension("OES_vertex_array_object")
-          .createVertexArrayOES().constructor;
+        let context = document.createElement('canvas').getContext('webgl');
+        if (context) {
+          window.WebGLVertexArrayObjectOES =
+              context.getExtension("OES_vertex_array_object")
+                     .createVertexArrayOES().constructor;
+        }
       }
     }
 
     // This import is only needed for chrome debugging. We should provide an
     // option to compile without it.
     dart_sdk._debugger.registerDevtoolsFormatter();
   }
 
 })(dart_library);