Gracefully handle pubspecs with dependencies using unknown sources.

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 527 matching lines @@
 }
 
 /// Converts a list of dependencies as passed to [package] into a hash as used
 /// in a pubspec.
 Future<Map> dependencyListToMap(List<Map> dependencies) {
   return awaitObject(dependencies).then((resolvedDependencies) {
     var result = <String, Map>{};
     for (var dependency in resolvedDependencies) {
       var keys = dependency.keys.where((key) => key != "version");
       var sourceName = only(keys);
-      var source;
-      switch (sourceName) {
-      case "git":
-        source = new GitSource();
-        break;
-      case "hosted":
-        source = new HostedSource();
-        break;
-      case "path":
-        source = new PathSource();
-        break;
-      default:
-        throw new Exception('Unknown source "$sourceName"');
-      }
 
       result[_packageName(sourceName, dependency[sourceName])] = dependency;
     }
     return result;
   });
 }
 
 /// Return the name for the package described by [description] and from
 /// [sourceName].
 String _packageName(String sourceName, description) {
@@ skipping 124 matching lines @@
   bool matches(item, MatchState matchState) {
     if (item is! Pair) return false;
     return _firstMatcher.matches(item.first, matchState) &&
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }