Allow dart2dart to output one file per library.

sdk/lib/_internal/compiler/implementation/util/util.dart

 // Copyright (c) 2011, 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 dart2js.util;
 
 import 'util_implementation.dart';
 import 'characters.dart';
 
 export 'setlet.dart';
@@ skipping 147 matching lines @@
   if (isAbstract) builder.addLast('abstract');
   if (isFinal) builder.addLast('final');
   if (isVar) builder.addLast('var');
   if (isConst) builder.addLast('const');
   if (isFactory) builder.addLast('factory');
   if (isExternal) builder.addLast('external');
   StringBuffer buffer = new StringBuffer();
   builder.toLink().printOn(buffer, ', ');
   return buffer.toString();
 }
+
+// Returns suggestedName if it is not in usedNames. Otherwise concatenates
+// the smallest number that makes it not appear in usedNames.
+// Adds the result to usedNames.
+String makeUnique(String suggestedName, Set<String> usedNames) {
+  String result = suggestedName;
+  if (usedNames.contains(suggestedName)) {
+    int counter = 0;
+    while (usedNames.contains(result)) {
+      counter++;
+      result = "$suggestedName$counter";
+    }
+  }
+  usedNames.add(result);
+  return result;
+}