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

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index c8faf136a6c2544c1419e540ba633c7dd3e72f3c..d564850c8b2c1ef8cc80a70f95115f24f70e9b3d 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -705,10 +705,12 @@ class ContextManagerImpl implements ContextManager {
     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);
+        }
       }
     }
   }