Allow user to specify mode for pub build/serve.

sdk/lib/_internal/pub/test/serve/allows_arbitrary_modes_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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.
 
 library pub_tests;
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import '../serve/utils.dart';
 
 const TRANSFORMER = """
 import 'dart:async';
 
 import 'package:barback/barback.dart';
+import 'package:source_maps/source_maps.dart';
 
-class RewriteTransformer extends Transformer {
-  RewriteTransformer.asPlugin();
+class ModeTransformer extends Transformer {
+  final BarbackSettings settings;
+  ModeTransformer.asPlugin(this.settings);
 
   String get allowedExtensions => '.txt';
 
   Future apply(Transform transform) {
-    return transform.readInputAsString(transform.primaryInput.id)
-        .then((contents) {
+    return new Future.value().then((_) {
       var id = transform.primaryInput.id.changeExtension(".out");
-      transform.addOutput(new Asset.fromString(id, "\$contents.out"));
+      transform.addOutput(new Asset.fromString(id, settings.mode.toString()));
     });
   }
 }
 """;
 
 main() {
   initConfig();
-  integration("a transform can use readInputAsString", () {
+  integration("allows user-defined mode names", () {
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
         "transformers": ["myapp/src/transformer"]
       }),
       d.dir("lib", [d.dir("src", [
         d.file("transformer.dart", TRANSFORMER)
       ])]),
       d.dir("web", [
         d.file("foo.txt", "foo")
       ])
     ]).create();
 
     createLockFile('myapp', pkg: ['barback']);
 
-    pubServe();
-    requestShouldSucceed("foo.out", "foo.out");
+    pubServe(args: ["--mode", "depeche"]);
+    requestShouldSucceed("foo.out", "depeche");
     endPubServe();
   });
 }