Analysis request `getReachableSources` (#24893).

pkg/analysis_server/test/services/dependencies/library_dependencies_test.dart

 // Copyright (c) 2015, 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.services.dependencies.library;
 
 import 'package:analysis_server/src/services/dependencies/library_dependencies.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 16 matching lines @@
     provider.newFile('/lib6.dart', '');
 
     _performAnalysis();
 
     var libs =
         new LibraryDependencyCollector([context]).collectLibraryDependencies();
 
     // Cycles
     expect(libs, contains('/lib1.dart'));
     expect(libs, contains('/lib2.dart'));
-    // Regular sourcs
+    // Regular sources
     expect(libs, contains('/lib3.dart'));
     expect(libs, contains('/lib4.dart'));
     // Non-source, referenced by source
     expect(libs, contains('/lib5.dart'));
     // Non-source, referenced by non-source
     expect(libs, contains('/lib6.dart'));
   }
 
   test_PackageMaps() {
     //TODO(pquitslund): add test
   }
 
   void _performAnalysis() {
     while (context.performAnalysisTask().hasMoreWork);
   }
 }