Check the type of a map entry before using it (issue 28756)

pkg/analysis_server/lib/src/context_manager.dart

 // Copyright (c) 2014, 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 context.directory.manager;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:core';
@@ skipping 687 matching lines @@
     info.context.analysisOptions = analysisOptions;
 
     // Nothing more to do.
     if (options == null) {
       return;
     }
 
     var analyzer = options[AnalyzerOptions.analyzer];
     if (analyzer is Map) {
       // Set ignore patterns.
-      YamlList exclude = analyzer[AnalyzerOptions.exclude];
-      List<String> excludeList = toStringList(exclude);
-      if (excludeList != null) {
-        setIgnorePatternsForContext(info, excludeList);
+      var exclude = analyzer[AnalyzerOptions.exclude];
+      if (exclude is YamlList) {
+        List<String> excludeList = toStringList(exclude);
+        if (excludeList != null) {
+          setIgnorePatternsForContext(info, excludeList);
+        }
       }
     }
   }
 
   /**
    * Process [options] for the given context [info].
    */
   void processOptionsForDriver(ContextInfo info,
       AnalysisOptionsImpl analysisOptions, Map<String, Object> options) {
     if (options == null) {
@@ skipping 1217 matching lines @@
     }
     return _embedderLocator;
   }
 
   @override
   SdkExtensionFinder getSdkExtensionFinder(ResourceProvider resourceProvider) {
     return _sdkExtensionFinder ??=
         new SdkExtensionFinder(buildPackageMap(resourceProvider));
   }
 }