Cache output of dart2js compilations that went wrong on disk.

tools/testing/dart/test_suite.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.
 
 /**
  * Classes and methods for enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 1899 matching lines @@
 
   /**
    * Copy a [source] file to a new place.
    * Assumes that the directory for [dest] already exists.
    */
   static Future copyFile(Path source, Path dest) {
     return new File(source.toNativePath()).openRead()
         .pipe(new File(dest.toNativePath()).openWrite());
   }
 
+  static Future isContentEqual(Path source, Path dest) {
+    return new File(source.toNativePath()).readAsString()
+      .then((srcString) {
+        return new File(dest.toNativePath()).readAsString()
+          .then((destString) => srcString == destString);
+      })
+      .catchError((error) {
+          return false;
+      });
+  }
+
+  static Future copyIfDifferent(Path source, Path dest) {
+    return isContentEqual(source, dest).then((equal) {
+      return equal ? null : TestUtils.copyFile(source, dest);
+    });
+  }
+
   static Future copyDirectory(String source, String dest) {
     source = new Path(source).toNativePath();
     dest = new Path(dest).toNativePath();
 
     var executable = 'cp';
     var args = ['-Rp', source, dest];
     if (Platform.operatingSystem == 'windows') {
       executable = 'xcopy';
       args = [source, dest, '/e', '/i'];
     }
@@ skipping 210 matching lines @@
         path.length > WINDOWS_SHORTEN_PATH_LIMIT) {
       for (var key in PATH_REPLACEMENTS.keys) {
         if (path.startsWith(key)) {
           path = path.replaceFirst(key, PATH_REPLACEMENTS[key]);
           break;
         }
       }
     }
     return path;
   }
+
+  static String cachedOutputFile(String outputFile) {
+    return "$outputFile.cached_output";
+  }
+
 }
 
 class SummaryReport {
   static int total = 0;
   static int skipped = 0;
   static int skippedByDesign = 0;
   static int noCrash = 0;
   static int pass = 0;
   static int failOk = 0;
   static int fail = 0;
@@ skipping 51 matching lines @@
  * $pass tests are expected to pass
  * $failOk tests are expected to fail that we won't fix
  * $fail tests are expected to fail that we should fix
  * $crash tests are expected to crash that we should fix
  * $timeout tests are allowed to timeout
  * $compileErrorSkip tests are skipped on browsers due to compile-time error
 """;
     print(report);
   }
 }