Fix failure using custom drivers with AnalysisDriverScheduler

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:typed_data';
 
 import 'package:analyzer/context/context_root.dart';
 import 'package:analyzer/context/declared_variables.dart';
@@ skipping 1349 matching lines @@
     }
     return false;
   }
 
   /**
    * Add the given [driver] and schedule it to perform its work.
    */
   void add(AnalysisDriverGeneric driver) {
     _drivers.add(driver);
     _hasWork.notify();
-    driverWatcher?.addedDriver(driver, driver.contextRoot);
+    if (driver is AnalysisDriver) {
+      driverWatcher?.addedDriver(driver, driver.contextRoot);
+    }
   }
 
   /**
    * Notify that there is a change to the [driver], it it might need to
    * perform some work.
    */
   void notify(AnalysisDriverGeneric driver) {
     _hasWork.notify();
     _statusSupport.preTransitionToAnalyzing();
   }
@@ skipping 625 matching lines @@
           libraryDeclarations.add(new TopLevelDeclarationInSource(
               file.source, declaration, isExported));
         }
       }
     }
 
     // We're not done yet.
     return false;
   }
 }