Fix for top-level inference error in SourceLoader.

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index d3e8b7f9ac06440cc07dde160bbcafdbc6c6712f..87f814676de715a67063189b0ed89078ab5a013f 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -46,7 +46,7 @@ import 'source_library_builder.dart' show SourceLibraryBuilder;
 
 class SourceLoader<L> extends Loader<L> {
   final Map<Uri, List<int>> sourceBytes = <Uri, List<int>>{};
-  final excludeSource = CompilerContext.current.options.excludeSource;
+  final bool excludeSource = CompilerContext.current.options.excludeSource;
 
   // Used when building directly to kernel.
   ClassHierarchy hierarchy;