Allow user to specify mode for pub build/serve.

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

Index: sdk/lib/_internal/pub/test/serve/allows_arbitrary_modes_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/can_use_read_input_as_string_test.dart b/sdk/lib/_internal/pub/test/serve/allows_arbitrary_modes_test.dart
similarity index 69%
copy from sdk/lib/_internal/pub/test/transformer/can_use_read_input_as_string_test.dart
copy to sdk/lib/_internal/pub/test/serve/allows_arbitrary_modes_test.dart
index 1484c769d29c980fe217af314067868f9b15e3c8..7d6be6c4b7b90e9b2d610e53bcd51374241b8881 100644
--- a/sdk/lib/_internal/pub/test/transformer/can_use_read_input_as_string_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/allows_arbitrary_modes_test.dart
@@ -12,17 +12,18 @@ 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()));
     });
   }
 }
@@ -30,7 +31,7 @@ class RewriteTransformer extends Transformer {
 
 main() {
   initConfig();
-  integration("a transform can use readInputAsString", () {
+  integration("allows user-defined mode names", () {
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
@@ -46,8 +47,8 @@ main() {
 
     createLockFile('myapp', pkg: ['barback']);
 
-    pubServe();
-    requestShouldSucceed("foo.out", "foo.out");
+    pubServe(args: ["--mode", "depeche"]);
+    requestShouldSucceed("foo.out", "depeche");
     endPubServe();
   });
 }