Make unittests work on Windows (again)

tests/compiler/dart2js/source_map_pub_build_validity_test.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:async_helper/async_helper.dart';
-
+import 'package:expect/expect.dart';
 import 'source_map_validator_helper.dart';
 
 void main() {
   asyncTest(() => createTempDir().then((Directory tmpDir) {
     Directory sunflowerDir = new Directory.fromUri(
         Platform.script.resolve('../../../third_party/sunflower'));
 
     print("Copying '${sunflowerDir.path}' to '${tmpDir.path}'.");
     copyDirectory(sunflowerDir, tmpDir);
     String ext = Platform.isWindows ? '.bat' : '';
     String command = path.normalize(path.join(path.fromUri(Platform.script),
                                               '../../../../sdk/bin/pub${ext}'));
     String file = path.join(tmpDir.path, 'build/web/sunflower.dart.js');
     print("Running '$command build --mode=debug' from '${tmpDir}'.");
     return Process.run(command, ['build','--mode=debug'],
-        workingDirectory: tmpDir.path).then((process) {
-      print(process.stdout);
+        workingDirectory: tmpDir.path).then((ProcessResult processResult) {
+      print(processResult.stdout);
+      print(processResult.stderr);
+      Expect.equals(0, processResult.exitCode, 'Unexpected exitCode from pub');
       validateSourceMap(new Uri.file(file, windows: Platform.isWindows));
       print("Deleting '${tmpDir.path}'.");
       tmpDir.deleteSync(recursive: true);
     });
   }));
 }