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

lib/runtime/dart_library.js

Index: lib/runtime/dart_library.js
diff --git a/lib/runtime/dart_library.js b/lib/runtime/dart_library.js
index d7ac9a3c271ed2c0e6a731e33e6f0f4ce8d95de9..134c948ac175358086269a42f60bf1316181f669 100644
--- a/lib/runtime/dart_library.js
+++ b/lib/runtime/dart_library.js
@@ -139,10 +139,12 @@ var dart_library =
         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;
+        }
       }
     }