Add a "global activate" command to pub.

sdk/lib/_internal/pub/test/global/activate/different_version_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 'package:scheduled_test/scheduled_test.dart';
+
+import '../../descriptor.dart' as d;
+import '../../test_pub.dart';
+
+main() {
+  initConfig();
+  integration("discards the previous active version if it does't match the "

[nweiz, 2014/06/26 22:43:26]

"does't" -> "doesn't"

[Bob Nystrom, 2014/06/27 00:30:23]

Do'ne.

+      "constraint", () {
+    servePackages([
+      packageMap("foo", "1.0.0"),
+      packageMap("foo", "2.0.0")
+    ]);
+
+    // Activate 1.2.3.

[nweiz, 2014/06/26 22:43:26]

"1.2.3" -> "1.0.0"

[Bob Nystrom, 2014/06/27 00:30:23]

Done.

+    schedulePub(args: ["global", "activate", "foo", "1.0.0"]);
+
+    // Activating it again with a different constraint changes the version.
+    schedulePub(args: ["global", "activate", "foo", ">1.0.0"], output: """
+Package foo is already active at version 1.0.0.
+Downloading foo 2.0.0...
+Resolving dependencies...
+Activated foo 2.0.0.""");
+  });
+}