Use a shared function to represent type substitutions with dynamic.

pkg/compiler/lib/src/js_emitter/code_emitter_task.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 dart2js.js_emitter;
 
 const USE_NEW_EMITTER = const bool.fromEnvironment("dart2js.use.new.emitter");
 
 /**
  * Generates the code for all used classes in the program. Static fields (even
@@ skipping 440 matching lines @@
 
   /// Returns the JS prototype of the given class [e].
   jsAst.Expression prototypeAccess(ClassElement e, bool hasBeenInstantiated);
 
   /// Returns the JS constructor of the given interceptor class [e].
   jsAst.Expression interceptorClassAccess(ClassElement e);
 
   /// Returns the JS expression representing the type [e].
   jsAst.Expression typeAccess(Element e);
 
+  /// Returns the JS expression representing a function that returns 'null'
+  jsAst.Expression generateFunctionThatReturnsNull();
+
   int compareConstants(ConstantValue a, ConstantValue b);
   bool isConstantInlinedOrAlreadyEmitted(ConstantValue constant);
 
   void invalidateCaches();
 }