Add Folder.contains(path) and use it in ContextDirectoryManager.

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index ee32ada5704a3ce21ea7ccae4e72d87cdb31ebb9..cb939aa467dfe7f4567ca9ae02d308a5da1fcd79 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -290,6 +290,11 @@ class _MemoryFolder extends _MemoryResource implements Folder {
   }
 
   @override
+  bool contains(String path) {
+    return posix.isWithin(this.path, path);
+  }
+
+  @override
   Resource getChild(String relPath) {
     String childPath = canonicalizePath(relPath);
     _MemoryResource resource = _provider._pathToResource[childPath];