Fix error reporting on invalid command line args.

utils/tests/pub/version_solver_test.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.
 
 library pub_update_test;
 
 import 'dart:io';
 import 'dart:isolate';
 
 import '../../pub/lock_file.dart';
 import '../../pub/package.dart';
 import '../../pub/pubspec.dart';
 import '../../pub/root_source.dart';
 import '../../pub/source.dart';
 import '../../pub/source_registry.dart';
 import '../../pub/system_cache.dart';
 import '../../pub/utils.dart';
 import '../../pub/version.dart';
 import '../../pub/version_solver.dart';
 import '../../../pkg/unittest/lib/unittest.dart';
 
 final noVersion = 'no version';
 final disjointConstraint = 'disjoint';
 final sourceMismatch = 'source mismatch';
 final descriptionMismatch = 'description mismatch';
 final couldNotSolve = 'unsolved';
 
-Source source1;
-Source source2;
+MockSource source1;
+MockSource source2;
 Source versionlessSource;
 Source rootSource;
 
 main() {
   testResolve('no dependencies', {
     'myapp 0.0.0': {}
   }, result: {
     'myapp from root': '0.0.0'
   });
 
@@ skipping 499 matching lines @@
   var match = new RegExp(r"(.*) from (.*)").firstMatch(name);
   if (match == null) return new Pair<String, Source>(name, source1);
   switch (match[2]) {
   case 'mock1': return new Pair<String, Source>(match[1], source1);
   case 'mock2': return new Pair<String, Source>(match[1], source2);
   case 'root': return new Pair<String, Source>(match[1], rootSource);
   case 'versionless':
     return new Pair<String, Source>(match[1], versionlessSource);
   }
 }