Add a --mode flag to pub run and pub global run.

sdk/lib/_internal/pub/test/global/run/mode_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 '../../descriptor.dart' as d;
+import '../../test_pub.dart';
+
+const TRANSFORMER = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class DartTransformer extends Transformer {
+  final BarbackSettings _settings;
+
+  DartTransformer.asPlugin(this._settings);
+
+  String get allowedExtensions => '.in';
+
+  void apply(Transform transform) {
+    transform.addOutput(new Asset.fromString(
+        new AssetId(transform.primaryInput.id.package, "bin/script.dart"),
+        "void main() => print('\${_settings.mode.name}');"));
+  }
+}
+""";
+
+main() {
+  initConfig();
+  integration('runs a script in an activated package with customizable modes',
+      () {
+    servePackages((builder) {
+      builder.serveRepoPackage("barback");
+
+      builder.serve("foo", "1.0.0",
+          deps: {"barback": "any"},
+          pubspec: {"transformers": ["foo/src/transformer"]},
+          contents: [
+        d.dir("lib", [d.dir("src", [
+          d.file("transformer.dart", TRANSFORMER),
+          d.file("primary.in", "")
+        ])])
+      ]);
+    });
+
+    schedulePub(args: ["global", "activate", "foo"]);
+
+    // By default it should run in release mode.
+    var pub = pubRun(global: true, args: ["foo:script"]);
+    pub.stdout.expect("release");
+    pub.shouldExit();
+
+    // A custom mode should be specifiable.
+    pub = pubRun(global: true, args: ["--mode", "custom-mode", "foo:script"]);
+    pub.stdout.expect("custom-mode");
+    pub.shouldExit();
+  });
+}