Change default binary file extension to .dill

lib/src/run_batch.dart

 // Copyright (c) 2016, 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.md file.
 
 library rasta.run_batch;
 
 import 'dart:async' show
     Future,
     Stream;
 
@@ skipping 67 matching lines @@
           ? readTestsFromStream(inputFile.openRead())
           : listTests(<Uri>[testUri], pattern: globalOptions.pattern);
     }
 
     await for (TestDescription description in tests) {
       assert(kernel.isInternalStateConsistent);
       count++;
 
       int failuresBefore = compiler.elementsWithCompileTimeErrors.length;
       Uri output = description.output;
-      output ??= description.uri.resolve("${description.uri.path}.bart");
+      output ??= description.uri.resolve("${description.uri.path}.dill");
 
       Options options = new Options(
           description.uri,
           output,
           dependenciesFile: null,
           generateLibrary: globalOptions.generateLibrary,
           isVerbose: globalOptions.isVerbose || batchFromStdin,
           isBatch: false,
           pattern: null);
 
@@ skipping 50 matching lines @@
 Failures: ${failures.length}
 Average time per test: ${averageTimeMs}ms
 Success rate: ${(100 - failures.length * 100/count).toStringAsFixed(2)}%""");
   }
 }
 
 String padTo(int i, int pad) {
   String result = (" " * pad) + "$i";
   return result.substring(result.length - pad);
 }