Rework Unparser/unparse StringBuffer management.

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 DartBackend extends Backend {
   final List<CompilerTask> tasks;
   final bool cutDeclarationTypes;
   // TODO(antonm): make available from command-line options.
   final bool outputAst = false;
 
@@ skipping 163 matching lines @@
         } else {
           outputElement(topLevel);
         }
       }
       sb.add('</Program>\n');
       compiler.assembledCode = sb.toString();
       return;
     }
 
     final unparser = new Unparser.withRenamer((Node node) => renames[node]);
-    compiler.assembledCode = emitCode(
-        compiler, unparser, imports, sortedTopLevels, sortedClassMembers);
+    emitCode(compiler, unparser, imports, sortedTopLevels, sortedClassMembers);
+    compiler.assembledCode = unparser.result;
   }
 
   log(String message) => compiler.log('[DartBackend] $message');
 }
 
 /**
  * Some elements are not recorded by resolver now,
  * for example, typedefs or classes which are only
  * used in signatures, as/is operators or in super clauses
  * (just to name a few).  Retraverse AST to pick those up.
@@ skipping 50 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);