Add a "global activate" command to pub.

sdk/lib/_internal/pub/test/global/activate/different_version_test.dart

Index: sdk/lib/_internal/pub/test/global/activate/different_version_test.dart
diff --git a/sdk/lib/_internal/pub/test/global/activate/different_version_test.dart b/sdk/lib/_internal/pub/test/global/activate/different_version_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..42f4c6209cbc1400fe2f14589a66081801930db4
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/global/activate/different_version_test.dart
@@ -0,0 +1,29 @@
+// 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 doesn't match the "
+      "constraint", () {
+    servePackages([
+      packageMap("foo", "1.0.0"),
+      packageMap("foo", "2.0.0")
+    ]);
+
+    // Activate 1.0.0.
+    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.""");
+  });
+}