Rework Unparser/unparse StringBuffer management.

lib/compiler/implementation/tree/unparser.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.
 
 // Returns null if no need to rename a node.
 typedef String Renamer(Node node);
 
+String unparse(Node node) {
+  Unparser unparser = new Unparser();
+  unparser.visit(node);
+  return unparser.result;
+}
+
 class Unparser implements Visitor {
   Renamer rename;
-  StringBuffer sb;
+  final StringBuffer sb;
 
-  Unparser() {
+  String get result => sb.toString();
+
+  Unparser() : sb = new StringBuffer() {
     // TODO(smok): Move this to initializer once dart2js stops complaining
     // about closures in initializers.
     rename = (Node node) => null;
   }
-  Unparser.withRenamer(this.rename);
+  Unparser.withRenamer(this.rename) : sb = new StringBuffer();
 
-  String unparse(Node node) {
-    sb = new StringBuffer();
-    visit(node);
-    return sb.toString();
+  // TODO(antonm): will go away soon.
+  void addString(String s) {
+    sb.add(s);
   }
 
   void add(SourceString string) {
     string.printOn(sb);
   }
 
   void addToken(Token token) {
     if (token === null) return;
     add(token.value);
     if (token.kind === KEYWORD_TOKEN || token.kind === IDENTIFIER_TOKEN) {
@@ skipping 462 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }