Implement substitution for type variables.

sdk/lib/_internal/compiler/implementation/js_backend/backend.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.
 
 part of js_backend;
 
 typedef void Recompile(Element element);
 
 class ReturnInfo {
   HType returnType;
@@ skipping 1185 matching lines @@
     return compiler.findHelper(const SourceString('makeLiteralMap'));
   }
 
   Element getSetRuntimeTypeInfo() {
     return compiler.findHelper(const SourceString('setRuntimeTypeInfo'));
   }
 
   Element getGetRuntimeTypeInfo() {
     return compiler.findHelper(const SourceString('getRuntimeTypeInfo'));
   }
+
+  Element getGetRuntimeTypeArgument() {
+    return compiler.findHelper(const SourceString('getRuntimeTypeArgument'));
+  }
 }