Don't use firstWhere as it throws an error if there is no such element

pkg/analyzer/lib/src/generated/source.dart

Index: pkg/analyzer/lib/src/generated/source.dart
diff --git a/pkg/analyzer/lib/src/generated/source.dart b/pkg/analyzer/lib/src/generated/source.dart
index 2a06746b459a078d21db0bfb5e46958279563283..01bff35112ce33b3dbc55c0c9327085916c9afbe 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -606,7 +606,7 @@ class SourceFactory {
   /// the package (or [null] if there is no registered package URI resolver).
   Map<String, List<Folder>> get packageMap {
     PackageMapUriResolver resolver =
-        _resolvers.firstWhere((r) => r is PackageMapUriResolver, orElse: null);
+        _resolvers.firstWhere((r) => r is PackageMapUriResolver, orElse: () => null);
     return resolver != null ? resolver.packageMap : null;
   }