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

pkg/analyzer/lib/file_system/physical_file_system.dart

Index: pkg/analyzer/lib/file_system/physical_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/physical_file_system.dart b/pkg/analyzer/lib/file_system/physical_file_system.dart
index e43ef033c70e0f230a145c1597e53a0192ad96b2..3b256a96382dbc6aac88bf7b3ddc3b4db0f7d857 100644
--- a/pkg/analyzer/lib/file_system/physical_file_system.dart
+++ b/pkg/analyzer/lib/file_system/physical_file_system.dart
@@ -73,6 +73,11 @@ class _PhysicalFolder extends _PhysicalResource implements Folder {
   }
 
   @override
+  bool contains(String path) {
+    return posix.isWithin(this.path, path);
+  }
+
+  @override
   Resource getChild(String relPath) {
     String canonicalPath = canonicalizePath(relPath);
     return PhysicalResourceProvider.INSTANCE.getResource(canonicalPath);