Use a lockfile to persist Pub's installed version constellation.

utils/tests/pub/test_pub.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Test infrastructure for testing pub. Unlike typical unit tests, most pub
  * tests are integration tests that stage some stuff on the file system, run
  * pub, and then validate the results. This library provides an API to build
  * tests like that.
  */
@@ skipping 34 matching lines @@
 GitRepoDescriptor git(Pattern name, [List<Descriptor> contents]) =>
     new GitRepoDescriptor(name, contents);
 
 /**
  * Creates a new [TarFileDescriptor] with [name] and [contents].
  */
 TarFileDescriptor tar(Pattern name, [List<Descriptor> contents]) =>
     new TarFileDescriptor(name, contents);
 
 /**
+ * The current [HttpServer] created using [serve].
+ */
+var _server;
+
+/**
  * Creates an HTTP server to serve [contents] as static files. This server will
  * exist only for the duration of the pub run.
+ *
+ * Subsequent calls to [serve] will replace the previous server.
  */
 void serve(String host, int port, [List<Descriptor> contents]) {
   var baseDir = dir("serve-dir", contents);
   if (host == 'localhost') {
     host = '127.0.0.1';
   }
 
   _schedule((_) {
-    var server = new HttpServer();
-    server.defaultRequestHandler = (request, response) {
+    if (_server != null) _server.close();
+    _server = new HttpServer();
+    _server.defaultRequestHandler = (request, response) {
       var path = request.uri.replaceFirst("/", "").split("/");
       response.persistentConnection = false;
       var stream;
       try {
         stream = baseDir.load(path);
       } catch (var e) {
         response.statusCode = 404;
         response.contentLength = 0;
         response.outputStream.close();
         return;
       }
 
       var future = consumeInputStream(stream);
       future.then((data) {
         response.statusCode = 200;
         response.contentLength = data.length;
         response.outputStream.write(data);
         response.outputStream.close();
       });
 
       future.handleException((e) {
         print("Exception while handling ${request.uri}: $e");
         response.statusCode = 500;
         response.reasonPhrase = e.message;
         response.outputStream.close();
       });
     };
-    server.listen(host, port);
-    _scheduleCleanup((_) => server.close());
+    _server.listen(host, port);
+    _scheduleCleanup((_) {
+      if (_server != null) _server.close();
+      _server = null;
+    });
 
     return new Future.immediate(null);
   });
 }
 
 /**
  * Creates an HTTP server that replicates the structure of pub.dartlang.org.
  * [pubspecs] is a list of YAML-format pubspecs representing the packages to
  * serve.
  */
@@ skipping 678 matching lines @@
 }
 
 /**
  * Schedules a callback to be called after Pub is run with [runPub], even if it
  * fails.
  */
 void _scheduleCleanup(_ScheduledEvent event) {
   if (_scheduledCleanup == null) _scheduledCleanup = [];
   _scheduledCleanup.add(event);
 }