Fixed dart2js compile errors in ir_builder and ir_nodes.

sdk/lib/_internal/compiler/implementation/ir/ir_nodes.dart

 // Copyright (c) 2013, 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.
 
 // IrNodes are kept in a separate library to have precise control over their
 // dependencies on other parts of the system.
 library dart2js.ir_nodes;
 
 import '../dart2jslib.dart' as dart2js show Constant, ConstructedConstant;
 import '../elements/elements.dart'
@@ skipping 292 matching lines @@
   Constant(this.value);
 
   dart2js.Constant get constant => value;
 
   accept(Visitor visitor) => visitor.visitConstant(this);
 }
 
 class This extends Primitive {
   This();
 
+  dart2js.Constant get constant => null;
+
   accept(Visitor visitor) => visitor.visitThis(this);
 }
 
 class LiteralList extends Primitive {
   /// The List type being created; this is not the type argument.
   final GenericType type;
   final List<Reference> values;
 
   /// Set to null if this is not a const literal list.
   final dart2js.Constant constant;
@@ skipping 358 matching lines @@
     for (int i = node.parameters.length - 1; i >= 0; --i) {
       release(node.parameters[i]);
     }
   }
 
   void visitIsTrue(IsTrue node) {
     visitReference(node.value);
   }
 
 }