Add a "global activate" command to pub.

sdk/lib/_internal/pub/test/global/activate/same_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('keeps previously activated version if it meets the constraint',
+        () {
+    servePackages([
+      packageMap("foo", "1.2.3"),
+      packageMap("foo", "1.3.0")
+    ]);
+
+    // Activate 1.2.3.
+    schedulePub(args: ["global", "activate", "foo", "1.2.3"]);
+
+    // Activating it again re-resolves but maintains the version.
+    schedulePub(args: ["global", "activate", "foo", ">1.0.0"], output: """
+Package foo is already active at version 1.2.3.
+Resolving dependencies...
+Activated foo 1.2.3.""");
+  });
+}