Fix failure using custom drivers with AnalysisDriverScheduler

pkg/analyzer/lib/src/dart/analysis/driver.dart

Index: pkg/analyzer/lib/src/dart/analysis/driver.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index c8309aa9f46ff8cd3f32ec7d5f459ee45e033334..3d90bd482beb7fee9a1790ab2f6d99be683c1480 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -1367,7 +1367,9 @@ class AnalysisDriverScheduler {
   void add(AnalysisDriverGeneric driver) {
     _drivers.add(driver);
     _hasWork.notify();
-    driverWatcher?.addedDriver(driver, driver.contextRoot);
+    if (driver is AnalysisDriver) {
+      driverWatcher?.addedDriver(driver, driver.contextRoot);
+    }
   }
 
   /**