Enable unique global renaming in dart2dart minification.

tests/compiler/dart2js/mirror_helper_unique_minification_test.dart

+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+import 'memory_compiler.dart' show compilerFor;
+import '../../../sdk/lib/_internal/compiler/implementation/apiimpl.dart' show
+    Compiler;
+import
+    '../../../sdk/lib/_internal/compiler/implementation/dart_backend/dart_backend.dart'
+show
+    DartBackend;
+
+main() {
+  testUniqueMinification();
+  testNoUniqueMinification();
+}
+
+Compiler runCompiler({useMirrorHelperLibrary: false, minify: false}) {
+  List<String> options = ['--output-type=dart'];
+  if (minify) {
+    options.add('--minify');
+  }
+  Compiler compiler = compilerFor(MEMORY_SOURCE_FILES, options: options);
+  DartBackend backend = compiler.backend;
+  backend.useMirrorHelperLibrary = useMirrorHelperLibrary;
+  compiler.runCompiler(Uri.parse('memory:main.dart'));
+  return compiler;
+}
+
+void testUniqueMinification() {
+  Compiler compiler = runCompiler(useMirrorHelperLibrary: true, minify: true);
+  DartBackend backend = compiler.backend;
+  Map<Node, String> renames = backend.renames;
+
+  //'Foo' appears twice, so toSet() reduces the length by 1.
+  Expect.equals(renames.values.toSet().length, renames.values.length - 1);
+}
+
+void testNoUniqueMinification() {
+  Compiler compiler = runCompiler(useMirrorHelperLibrary: false, minify: true);
+  DartBackend backend = compiler.backend;
+  Map<Node, String> renames = backend.renames;
+
+  //'Foo' appears twice and now 'invocation' and 'hest' can get the same name.
+  Expect.equals(renames.values.toSet().length, renames.values.length - 2);
+}
+
+const MEMORY_SOURCE_FILES = const <String, String> {
+  'main.dart': """
+import 'dart:mirrors';
+
+class Foo {
+  noSuchMethod(invocation) {
+    MirrorSystem.getName(const Symbol('hest'));
+  }
+}
+
+main() {
+  new Foo().fisk();
+  var hest;
+}
+"""};