Cache output of dart2js compilations that went wrong on disk.

tools/testing/dart/multitest.dart

Index: tools/testing/dart/multitest.dart
===================================================================
--- tools/testing/dart/multitest.dart	(revision 35513)
+++ tools/testing/dart/multitest.dart	(working copy)
@@ -248,9 +248,13 @@
     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)));
+    Future copy = TestUtils.isContentEqual(srcPath, targetPath).then((equal) {

[Bill Hesse, 2014/04/29 17:51:55]

It seems to me that "copyIfDifferent" is a useful function that could be added
to TestUtils, encapsulating this.

[ricow1, 2014/04/30 06:51:01]

Done.

+      return equal ? null : TestUtils.copyFile(srcPath, targetPath);
+    });
+    futureCopies.add(copy);
   }
 
   // Wait until all imports are copied before scheduling test cases.