Output test results in diff mode.

tools/migration/bin/run_tests.dart

 // Copyright (c) 2017, 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.
 
 /// Runs the tests in a batch on the various configurations used on the bots.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:args/args.dart';
@@ skipping 118 matching lines @@
     var path = parts.skip(1).join("/");
     selectors.putIfAbsent(selector, () => []).add(path);
   }
 
   var failed = <String>[];
   var passed = <String>[];
   for (var name in configs) {
     var configArgs = allConfigs[name];
     print("${bold(name)} ${configArgs.join(' ')}:");
 
-    var args = [
-      "--progress=color",
-    ];
+    var args = ["--progress=diff"];
 
     args.addAll(configArgs);
 
     if (!args.any((arg) => arg.startsWith("--mode"))) {
       args.add("--mode=release");
     }
 
     selectors.forEach((selector, paths) {
       args.add("$selector/${paths.join('|')}");
     });
@@ skipping 43 matching lines @@
   process.stdout.listen((bytes) {
     stdout.add(bytes);
   });
 
   process.stderr.listen((bytes) {
     stderr.add(bytes);
   });
 
   return await process.exitCode;
 }