Simplify PackageMapUriResolver - one folder, return any file, or null.

pkg/analysis_server/test/context_manager_test.dart

 // Copyright (c) 2014, 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 test.context.directory.manager;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/context_manager.dart';
 import 'package:analysis_server/src/utilities/null_string_sink.dart';
@@ skipping 422 matching lines @@
       List<AnalysisDriver> drivers = manager
           .getDriversInAnalysisRoot(resourceProvider.newFolder(projPath));
       expect(drivers, hasLength(1));
       expect(drivers[0], isNotNull);
     } else {
       List<AnalysisContext> contextsInAnalysisRoot =
           manager.contextsInAnalysisRoot(resourceProvider.newFolder(projPath));
       expect(contextsInAnalysisRoot, hasLength(1));
       expect(contextsInAnalysisRoot[0], isNotNull);
     }
-    Source result = sourceFactory.forUri('package:foo/foo.dart');
+    Source result = sourceFactory.forUri('dart:async');
     expect(result, isNotNull);
-    expect(result.exists(), isFalse);
   }
 
   void test_setRoots_addFolderWithDartFileInSubfolder() {
     String filePath = path.posix.join(projPath, 'foo', 'bar.dart');
     resourceProvider.newFile(filePath, 'contents');
     manager.setRoots(<String>[projPath], <String>[], <String, String>{});
     // verify
     Iterable<String> filePaths = callbacks.currentFilePaths;
     expect(filePaths, hasLength(1));
     expect(filePaths, contains(filePath));
@@ skipping 2444 matching lines @@
 class TestUriResolver extends UriResolver {
   Map<Uri, Source> uriMap;
 
   TestUriResolver(this.uriMap);
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     return uriMap[uri];
   }
 }