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

pkg/analysis_server/test/context_manager_test.dart

Index: pkg/analysis_server/test/context_manager_test.dart
diff --git a/pkg/analysis_server/test/context_manager_test.dart b/pkg/analysis_server/test/context_manager_test.dart
index 43026ebf223d5fc001bb0e420fc52077bbdfc387..c928026a38eeef69fcca3ed6e59dfe81d4a1fe3b 100644
--- a/pkg/analysis_server/test/context_manager_test.dart
+++ b/pkg/analysis_server/test/context_manager_test.dart
@@ -440,9 +440,8 @@ test_pack:lib/''');
       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() {