Implement references to "this" in the new IR.

sdk/lib/_internal/compiler/implementation/dart_backend/dart_codegen.dart

 // Copyright (c) 2014, 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 dart_codegen;
 
 import 'dart_tree.dart' as tree;
 import 'dart_printer.dart';
 import 'dart_tree_printer.dart' show TreePrinter;
 import '../tree/tree.dart' as frontend;
@@ skipping 315 matching lines @@
     statementBuffer = savedBuffer;
     fallthrough = savedFallthrough;
 
     visitStatement(stmt.next);
   }
 
   Expression visitConstant(tree.Constant exp) {
     return emitConstant(exp.value);
   }
 
+  Expression visitThis(tree.This exp) {
+    return new This();
+  }
+
   Expression visitLiteralList(tree.LiteralList exp) {
     return new LiteralList(
         exp.values.map(visitExpression).toList(growable: false),
         isConst: exp.constant != null,
         typeArgument: emitOptionalType(exp.type.typeArguments.single));
   }
 
   Expression visitLiteralMap(tree.LiteralMap exp) {
     List<LiteralMapEntry> entries = new List<LiteralMapEntry>.generate(
         exp.values.length,
@@ skipping 165 matching lines @@
     } else if (constant is dart2js.TypeConstant) {
       GenericType type = constant.representedType;
       return new Identifier(type.name)
                  ..element = type.element;
     } else {
       throw "Unsupported constant: $constant";
     }
   }
 }