Gracefully handle a missing locked version in pub get.

sdk/lib/_internal/pub/test/test_pub.dart

 // Copyright (c) 2013, 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.
 library test_pub;
 
@@ skipping 200 matching lines @@
 
 /// A map from package names to parsed pubspec maps for those packages. This
 /// represents the packages currently being served by [servePackages], and is
 /// `null` if [servePackages] has not yet been called for this test.
 Map<String, List<Map>> _servedPackages;
 
 /// Creates an HTTP server that replicates the structure of pub.dartlang.org.
 /// [pubspecs] is a list of unserialized pubspecs representing the packages to
 /// serve.
 ///
-/// Subsequent calls to [servePackages] will add to the set of packages that
-/// are being served. Previous packages will continue to be served.
-void servePackages(List<Map> pubspecs) {
+/// If [replace] is false, subsequent calls to [servePackages] will add to the
+/// set of packages that are being served. Previous packages will continue to be
+/// served. Otherwise, the previous packages will no longer be served.
+void servePackages(List<Map> pubspecs, {bool replace: false}) {
   if (_servedPackages == null || _servedPackageDir == null) {
     _servedPackages = <String, List<Map>>{};
     _servedApiPackageDir = d.dir('packages', []);
     _servedPackageDir = d.dir('packages', []);
     serve([
       d.dir('api', [_servedApiPackageDir]),
       _servedPackageDir
     ]);
 
     currentSchedule.onComplete.schedule(() {
       _servedPackages = null;
       _servedApiPackageDir = null;
       _servedPackageDir = null;
     }, 'cleaning up served packages');
   }
 
   schedule(() {
     return awaitObject(pubspecs).then((resolvedPubspecs) {
+      if (replace) _servedPackages.clear();
+
       for (var spec in resolvedPubspecs) {
         var name = spec['name'];
         var version = spec['version'];
         var versions = _servedPackages.putIfAbsent(name, () => []);
         versions.add(spec);
       }
 
       _servedApiPackageDir.contents.clear();
       _servedPackageDir.contents.clear();
       for (var name in _servedPackages.keys) {
@@ skipping 656 matching lines @@
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     return description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }
 
 /// A [StreamMatcher] that matches multiple lines of output.
 StreamMatcher emitsLines(String output) => inOrder(output.split("\n"));