Add CodeOutput to prepare for emission without in-memory buffers.

pkg/compiler/lib/src/js/nodes.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;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 524 matching lines @@
 
   Expression withPosition(var sourcePosition, var endSourcePosition) =>
       super.withPosition(sourcePosition, endSourcePosition);
 }
 
 /// Wrap a CodeBuffer as an expression.
 class Blob extends Expression {
   // TODO(ahe): This class is an aid to convert everything to ASTs, remove when
   // not needed anymore.
 
-  final leg.CodeBuffer buffer;
+  final CodeBuffer buffer;
 
   Blob(this.buffer);
 
   accept(NodeVisitor visitor) => visitor.visitBlob(this);
 
   void visitChildren(NodeVisitor visitor) {}
 
   Blob _clone() => new Blob(buffer);
 
   int get precedenceLevel => PRIMARY;
@@ skipping 544 matching lines @@
 class Comment extends Statement {
   final String comment;
 
   Comment(this.comment);
 
   accept(NodeVisitor visitor) => visitor.visitComment(this);
   Comment _clone() => new Comment(comment);
 
   void visitChildren(NodeVisitor visitor) {}
 }