Make pub generate .packages file.

test/packages_file_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS d.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 d.file.
+
+import 'descriptor.dart' as d;
+import 'test_pub.dart';
+
+main() {
+  initConfig();
+  forBothPubGetAndUpgrade((command) {
+    integration('check that .packages is created', () {
+      servePackages((builder) {
+        builder.serve("foo", "1.2.3",
+            deps: {'baz': '2.2.2'}, contents: [d.dir("lib", [])]);
+        builder.serve("bar", "3.2.1", contents: [d.dir("lib", [])]);
+        builder.serve("baz", "2.2.2",
+            deps: {"bar": "3.2.1"}, contents: [d.dir("lib", [])]);
+      });
+
+      d.appDir({"foo": "1.2.3"}).create();
+
+      pubCommand(command);
+
+      d.dir(appPath, [d.packagesFile({
+          "foo": "1.2.3", "bar": "3.2.1", "baz": "2.2.2", "myapp": "0.0.0"})])
+       .validate();
+    });
+  });
+}
+