Make unittests work on Windows (again)

tests/compiler/dart2js/source_map_pub_build_validity_test.dart

Index: tests/compiler/dart2js/source_map_pub_build_validity_test.dart
diff --git a/tests/compiler/dart2js/source_map_pub_build_validity_test.dart b/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
index 46b671edac6ef68c4ad57df004e606334e0759ea..939f9a233d3afc4f776c88b98409e02c1cd0243a 100644
--- a/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
+++ b/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
@@ -6,7 +6,7 @@ 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() {
@@ -22,8 +22,10 @@ void main() {
     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);