Add a "global activate" command to pub.

sdk/lib/_internal/pub/test/global/activate/uncached_package_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('installs and activates the best version of a package', () {
+    servePackages([
+      packageMap("foo", "1.0.0"),
+      packageMap("foo", "1.2.3"),
+      packageMap("foo", "2.0.0-wildly.unstable")
+    ]);
+
+    schedulePub(args: ["global", "activate", "foo"], output: """
+Downloading foo 1.2.3...
+Resolving dependencies...
+Activated foo 1.2.3.
+    """);
+
+    // Should be in global package cache.
+    d.dir(cachePath, [
+      d.dir('global_packages', [
+        d.matcherFile('foo.lock', contains('1.2.3'))
+      ])
+    ]).validate();
+  });
+}