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 18081ab420bb6fdc2c7282148a75db51da4ca7e3..3a0b82be1c1bb726739f5e51c438d9fdd916d43d 100644
--- a/pkg/front_end/lib/src/fasta/loader.dart
+++ b/pkg/front_end/lib/src/fasta/loader.dart
@@ -112,9 +112,9 @@ ${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>[];