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

pkg/analysis_server/lib/src/context_directory_manager.dart

Index: pkg/analysis_server/lib/src/context_directory_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_directory_manager.dart b/pkg/analysis_server/lib/src/context_directory_manager.dart
index 367d00758741872d23770eb64f640931367cc037..a8d024e1c90047cc0ed5a574b662501fff926e21 100644
--- a/pkg/analysis_server/lib/src/context_directory_manager.dart
+++ b/pkg/analysis_server/lib/src/context_directory_manager.dart
@@ -247,7 +247,7 @@ abstract class ContextDirectoryManager {
   bool isInAnalysisRoot(String path) {
     // TODO(scheglov) check for excluded paths
     for (Folder root in _currentDirectoryInfo.keys) {
-      if (path.startsWith(root.path)) {
+      if (root.contains(path)) {
         return true;
       }
     }