Guard against a hypothetical null pointer dereference

pkg/analysis_server/lib/src/status/memory_use.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.
 
 library analysis_server.src.status.memory_use;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 207 matching lines @@
    */
   void processAnalysisServer(AnalysisServer server) {
     _recordInstance(server, null);
     Iterable<AnalysisContext> contexts = server.analysisContexts;
     for (AnalysisContextImpl context in contexts) {
       _processAnalysisContext(context, server);
     }
     DartSdkManager manager = server.sdkManager;
     List<SdkDescription> descriptors = manager.sdkDescriptors;
     for (SdkDescription descriptor in descriptors) {
-      _processAnalysisContext(
-          manager.getSdk(descriptor, () => null).context, manager);
+      DartSdk sdk = manager.getSdk(descriptor, () => null);
+      if (sdk != null) {
+        _processAnalysisContext(sdk.context, manager);
+      }
     }
   }
 
   void _processAnalysisContext(AnalysisContextImpl context, Object owner) {
     _recordInstance(context, owner);
     _recordInstance(context.analysisCache, context);
     CachePartition partition = context.privateAnalysisCachePartition;
     Map<AnalysisTarget, CacheEntry> map = partition.entryMap;
     map.forEach((AnalysisTarget target, CacheEntry entry) {
       _processAnalysisTarget(target, partition);
@@ skipping 65 matching lines @@
       }
     }
     ownerMap
         .putIfAbsent(instance.runtimeType, () => new HashSet<Type>())
         .add(owner.runtimeType);
     if (instance is LibrarySpecificUnit) {
       uniqueLSUs.add(instance);
     }
   }
 }