Remove two more implicit upcasts with angular/dart analysis drivers

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 1431 matching lines @@
    * If the status is currently idle, the returned future will be signaled
    * immediately.
    */
   Future<Null> waitForIdle() => _statusSupport.waitForIdle();
 
   /**
    * Remove the given [driver] from the scheduler, so that it will not be
    * asked to perform any new work.
    */
   void _remove(AnalysisDriverGeneric driver) {
-    driverWatcher?.removedDriver(driver);
+    if (driver is AnalysisDriver) {
+      driverWatcher?.removedDriver(driver);
+    }
+
     _drivers.remove(driver);
     _hasWork.notify();
   }
 
   /**
    * Run infinitely analysis cycle, selecting the drivers with the highest
    * priority first.
    */
   Future<Null> _run() async {
     Stopwatch timer = new Stopwatch()..start();
     PerformanceLogSection analysisSection;
     while (true) {
       // Pump the event queue.
       if (timer.elapsedMilliseconds > _MS_BEFORE_PUMPING_EVENT_QUEUE) {
         await _pumpEventQueue(_NUMBER_OF_EVENT_QUEUE_PUMPINGS);
         timer.reset();
       }
 
       await _hasWork.signal;
 
       // Transition to analyzing if there are files to analyze.
       if (_hasFilesToAnalyze) {
         _statusSupport.transitionToAnalyzing();
         analysisSection ??= _logger.enter('Analyzing');
       }
 
       // Find the driver with the highest priority.
       AnalysisDriverGeneric bestDriver;
       AnalysisDriverPriority bestPriority = AnalysisDriverPriority.nothing;
-      for (AnalysisDriver driver in _drivers) {
+      for (AnalysisDriverGeneric driver in _drivers) {
         AnalysisDriverPriority priority = driver.workPriority;
         if (priority.index > bestPriority.index) {
           bestDriver = driver;
           bestPriority = priority;
         }
       }
 
       // Transition to idle if no files to analyze.
       if (!_hasFilesToAnalyze) {
         _statusSupport.transitionToIdle();
@@ skipping 560 matching lines @@
           libraryDeclarations.add(new TopLevelDeclarationInSource(
               file.source, declaration, isExported));
         }
       }
     }
 
     // We're not done yet.
     return false;
   }
 }