Implement substitution for type variables.

sdk/lib/_internal/compiler/implementation/lib/foreign_helper.dart

 // Copyright (c) 2012, 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 _foreign_helper;
 
 /**
  * Emits a JavaScript code fragment parameterized by arguments.
  *
  * Hash characters `#` in the [codeTemplate] are replaced in left-to-right order
@@ skipping 115 matching lines @@
 
 /**
  * Creates an isolate and returns it.
  */
 dynamic JS_CREATE_ISOLATE() {}
 
 /**
  * Returns the prefix used for generated is checks on classes.
  */
 String JS_OPERATOR_IS_PREFIX() {}
+
+/**
+ * Returns the prefix used for generated type argument substitutions on classes.
+ */
+String JS_OPERATOR_AS_PREFIX() {}