dart2dart: Use implicit 'this' when generating Dart code.

sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.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_tree;
 
 import '../dart2jslib.dart' as dart2js;
 import '../elements/elements.dart';
 import '../universe/universe.dart';
 import '../ir/ir_nodes.dart' as ir;
@@ skipping 170 matching lines @@
   Constant(this.expression, this.value);
 
   Constant.primitive(dart2js.PrimitiveConstant primitiveValue)
       : expression = new PrimitiveConstExp(primitiveValue),
         value = primitiveValue;
 
   accept(ExpressionVisitor visitor) => visitor.visitConstant(this);
 }
 
 class This extends Expression {
-  accept(Visitor visitor) => visitor.visitThis(this);
+  accept(ExpressionVisitor visitor) => visitor.visitThis(this);
 }
 
 class ReifyTypeVar extends Expression {
   TypeVariableElement element;
 
   ReifyTypeVar(this.element);
 
-  accept(Visitor visitor) => visitor.visitReifyTypeVar(this);
+  accept(ExpressionVisitor visitor) => visitor.visitReifyTypeVar(this);
 }
 
 class LiteralList extends Expression {
   final GenericType type;
   final List<Expression> values;
 
   LiteralList(this.type, this.values);
 
   accept(ExpressionVisitor visitor) => visitor.visitLiteralList(this);
 }
@@ skipping 1775 matching lines @@
     }
   }
 
   /// Destructively updates each entry of [l] with the result of visiting it.
   void _rewriteList(List<Expression> l) {
     for (int i = 0; i < l.length; i++) {
       l[i] = visitExpression(l[i]);
     }
   }
 }