Cache output of dart2js compilations that went wrong on disk.

tools/testing/dart/multitest.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.
 
 library multitest;
 
 import "dart:async";
 import "dart:io";
 import "test_suite.dart";
 import "utils.dart";
@@ skipping 230 matching lines @@
   // Copy all the relative imports of the multitest.
   Set<String> importsToCopy = _findAllRelativeImports(filePath);
   List<Future> futureCopies = [];
   for (String relativeImport in importsToCopy) {
     Path importPath = new Path(relativeImport);
     // Make sure the target directory exists.
     Path importDir = importPath.directoryPath;
     if (!importDir.isEmpty) {
       TestUtils.mkdirRecursive(targetDir, importDir);
     }
+    Path srcPath = sourceDir.join(importPath);
+    Path targetPath = targetDir.join(importPath);
     // Copy file.
-    futureCopies.add(TestUtils.copyFile(sourceDir.join(importPath),
-                                        targetDir.join(importPath)));
+    futureCopies.add(TestUtils.copyIfDifferent(srcPath, targetPath));
   }
 
   // Wait until all imports are copied before scheduling test cases.
   return Future.wait(futureCopies).then((_) {
     String baseFilename = filePath.filenameWithoutExtension;
     for (String key in tests.keys) {
       final Path multitestFilename =
           targetDir.append('${baseFilename}_$key.dart');
       writeFile(multitestFilename.toNativePath(), tests[key]);
       Set<String> outcome = outcomes[key];
@@ skipping 32 matching lines @@
     // TestSuite.forDirectory.
     split.removeLast();
   }
   String path = '${generatedTestDir.path}/${split.last}';
   Directory dir = new Directory(path);
   if (!dir.existsSync()) {
     dir.createSync();
   }
   return new Path(new File(path).absolute.path);
 }