Enable unique global renaming in dart2dart minification.

sdk/lib/_internal/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.
 
 part of dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 391 matching lines @@
         classMembers[element].forEach(makePlaceholders);
       }
     }
     topLevelElements.forEach(makePlaceholders);
     // Create renames.
     bool shouldCutDeclarationTypes = forceStripTypes
         || (compiler.enableMinification
             && isSafeToRemoveTypeDeclarations(classMembers));
     renamePlaceholders(
         compiler, collector, renames, imports,
-        fixedMemberNames, shouldCutDeclarationTypes);
+        fixedMemberNames, shouldCutDeclarationTypes,
+        uniqueGlobalNaming: useMirrorHelperLibrary);
 
     // 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.
@@ skipping 139 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().canonicalUri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Iterable<Element> elements) =>
     sorted(elements, compareElements);