Throw a better exception for a modified analysis cache key

pkg/analysis_server/lib/src/domain_diagnostic.dart

 // Copyright (c) 2015, 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.
 
 library analysis_server.src.domain_diagnostic;
 
 import 'dart:collection';
 import 'dart:core' hide Resource;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
@@ skipping 45 matching lines @@
     if (context is AnalysisContextImpl) {
       workItems = _workItemCount(context);
       var cache = context.analysisCache;
       if (cache is AnalysisCache) {
         Set<AnalysisTarget> countedTargets = new HashSet<AnalysisTarget>();
         MapIterator<AnalysisTarget, CacheEntry> iterator = cache.iterator();
         while (iterator.moveNext()) {
           AnalysisTarget target = iterator.key;
           if (countedTargets.add(target)) {
             CacheEntry cacheEntry = iterator.value;
+            if (cacheEntry == null) {
+              throw new StateError(
+                  "mutated cache key detected: $target (${target.runtimeType})");
+            }
             if (target is Source) {
               if (cacheEntry.explicitlyAdded) {
                 explicitFiles++;
               } else {
                 implicitFiles++;
               }
             }
             // Caught exceptions.
             if (cacheEntry.exception != null) {
               exceptions.add(cacheEntry.exception.toString());
@@ skipping 12 matching lines @@
       String requestName = request.method;
       if (requestName == DIAGNOSTICS) {
         return computeDiagnostics(request);
       }
     } on RequestFailure catch (exception) {
       return exception.response;
     }
     return null;
   }
 }