watch for file system changes after context created

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index 5085d24016ecb552507c47e5bba293ece3f2dab4..275df09683006d8a60a4122d29cb5731a3a57847 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -478,9 +478,14 @@ abstract class ContextManager {
     info.changeSubscription = folder.changes.listen((WatchEvent event) {
       _handleWatchEvent(folder, info, event);
     });
-    UriResolver packageUriResolver = _computePackageUriResolver(folder, info);
-    info.context = addContext(folder, packageUriResolver);
-    info.context.name = folder.path;
+    try {
+      UriResolver packageUriResolver = _computePackageUriResolver(folder, info);
+      info.context = addContext(folder, packageUriResolver);
+      info.context.name = folder.path;
+    } catch (_) {
+      info.changeSubscription.cancel();
+      rethrow;
+    }
     return info;
   }