Enable is and as checks on non-ground types

lib/runtime/_errors.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 library encapsulates the core sdk errors that the runtime knows about.
  *
  */
 
 dart_library.library('dart_runtime/_errors', null, /* Imports */[
@@ skipping 11 matching lines @@
 
   function throwCastError(actual, type) {
     operations.throw(new _js_helper.CastErrorImplementation(actual, type));
   }
   exports.throwCastError = throwCastError;
 
   function throwAssertionError() {
     operations.throw(new core.AssertionError());
   }
   exports.throwAssertionError = throwAssertionError;
+
+  function throwNullValueError() {
+    // TODO(vsm): Per spec, we should throw an NSM here.  Technically, we ought
+    // to thread through method info, but that uglifies the code and can't
+    // actually be queried ... it only affects how the error is printed.
+    operations.throw(new core.NoSuchMethodError(null,
+      new core.Symbol('<Unexpected Null Value>'), null, null, null));
+  }
+  exports.throwNullValueError = throwNullValueError;
 });