Improves options validation type safety (#24885).

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' hide Resource;
@@ skipping 487 matching lines @@
       }
     });
 
     // In case options files are removed, revert to default options.
     if (optionsRemoved) {
       info.context.analysisOptions = new AnalysisOptionsImpl();
       return;
     }
 
     // Analysis options are processed 'in-line'.
-    YamlMap analyzer = options[AnalyzerOptions.analyzer];
-    if (analyzer == null) {
+    YamlNode analyzer = options[AnalyzerOptions.analyzer];
+    if (analyzer is! YamlMap) {
       // No options for analyzer.
       return;
     }
 
     configureContextOptions(info.context, options);
 
     // Set ignore patterns.
     YamlList exclude = analyzer[AnalyzerOptions.exclude];
     if (exclude != null) {
       setIgnorePatternsForContext(info, exclude);
@@ skipping 934 matching lines @@
           var path = resourceProvider.pathContext.fromUri(uri);
           packageMap[name] = <Folder>[resourceProvider.getFolder(path)];
         }
       });
       return <UriResolver>[new SdkExtUriResolver(packageMap)];
     } else {
       return const <UriResolver>[];
     }
   }
 }