Fix default value of stored output in skipping tests

tests/standalone/io/skipping_dart2js_compilations_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.
 
 /*
  * This test makes sure that the "skipping Dart2Js compilations if the output is
  * already up to date" feature does work as it should.
  * Therefore this test ensures that compilations are only skipped if the last
  * modified date of the output of a dart2js compilation is newer than
  *   - the the dart application to compile (including it's dependencies)
@@ skipping 47 matching lines @@
     }
     if (createJsDeps) {
       testJsDeps = _createFile(testJsDepsFilePath);
       var path = suite.TestUtils.absolutePath(new Path(tempDir.path))
           .append("test.dart");
       _writeToFile(testJsDeps, "file://$path");
     }
     if (createCachedOutput) {
       testCachedOutput = _createFile(testCachedOutputPath);
       var content =  JSON.encode({
-          'stdout': '',
-          'stderr': '',
+          'stdout': [],
+          'stderr': [],
           'exitCode': 0,
           'timedOut': false
         });
       _writeToFile(testCachedOutput, content);
     }
 
   }
 
   void cleanup() {
     if (testJs != null) testJs.deleteSync();
@@ skipping 193 matching lines @@
       cleanup();
       throw error;
     }).then((_) {
       cleanup();
     });
   }
   // We need to wait some time to make sure that the files we 'touch' get a
   // bigger timestamp than the old ones
   new Timer(new Duration(seconds: 1), touchFilesAndRunTests);
 }