Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/loader.dart

Index: pkg/front_end/lib/src/fasta/loader.dart
diff --git a/pkg/front_end/lib/src/fasta/loader.dart b/pkg/front_end/lib/src/fasta/loader.dart
index 95ee99bde79986488c4bae88d5b09ffab8e357ef..f602cfab511b2216da004c8e5bfa052c92aa8012 100644
--- a/pkg/front_end/lib/src/fasta/loader.dart
+++ b/pkg/front_end/lib/src/fasta/loader.dart
@@ -130,9 +130,9 @@ ${format(ms / libraryCount, 3, 12)} ms/compilation unit.""");
     });
   }
 
-  Future<Null> buildOutline(LibraryBuilder library);
+  Future<Null> buildOutline(covariant LibraryBuilder library);
 
-  Future<Null> buildBody(LibraryBuilder library, AstKind astKind);
+  Future<Null> buildBody(covariant LibraryBuilder library, AstKind astKind);
 
   List<InputError> collectCompileTimeErrors() {
     List<InputError> errors = <InputError>[];