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 'dart:collection';
 import 'util_implementation.dart';
 import 'characters.dart';
 
@@ skipping 176 matching lines @@
 
 int longestCommonPrefixLength(List a, List b) {
   int index = 0;
   for ( ; index < a.length && index < b.length; index++) {
     if (a[index] != b[index]) {
       break;
     }
   }
   return index;
 }
+
+/// 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;
+}