Migrate to new Unparse API for AST emission.

lib/compiler/implementation/dart_backend/backend.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.
 
 class ElementAst {
   final TreeElements treeElements;
 
   ElementAst(this.treeElements);
   ElementAst.forClassLike() : this.treeElements = new TreeElementMapping();
 }
@@ skipping 199 matching lines @@
     // Sort elements.
     final sortedTopLevels = sortElements(topLevelElements);
     final sortedClassMembers = new Map<ClassElement, List<Element>>();
     classMembers.forEach((classElement, members) {
       sortedClassMembers[classElement] = sortElements(members);
     });
 
     if (outputAst) {
       // TODO(antonm): Ideally XML should be a separate backend.
       // TODO(antonm): obey renames and minification, at least as an option.
-      StringBuffer sb = new StringBuffer();
-      sb.add('<Program>\n');
-      outputElement(element) {
-        sb.add(element.parseNode(compiler).toDebugString());
-      }
+      final unparser = new Unparser();
+      outputElement(element) { unparser.unparse(element.parseNode(compiler)); }
 
       // Emit XML for AST instead of the program.
       for (final topLevel in sortedTopLevels) {
         if (topLevel is ClassElement) {
           // TODO(antonm): add some class info.
           sortedClassMembers[topLevel].forEach(outputElement);
         } else {
           outputElement(topLevel);
         }
       }
-      sb.add('</Program>\n');
-      compiler.assembledCode = sb.toString();
+      compiler.assembledCode = '<Program>\n${unparser.result}</Program>\n';
       return;
     }
 
     final unparser = new Unparser.withRenamer((Node node) => renames[node]);
     emitCode(compiler, unparser, imports, sortedTopLevels, sortedClassMembers);
     compiler.assembledCode = unparser.result;
   }
 
   log(String message) => compiler.log('[DartBackend] $message');
 }
@@ skipping 56 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Collection<Element> elements) =>
     sorted(elements, compareElements);