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 35463be1b0cc405c296bbcc064c80afdca3465b8..7e0200c433210b8f6d5ec2565fbd33d1ef8bc832 100644
--- a/pkg/front_end/lib/src/fasta/loader.dart
+++ b/pkg/front_end/lib/src/fasta/loader.dart
@@ -130,13 +130,13 @@ ${format(ms / libraryCount, 3, 12)} ms/compilation unit.""");
     });
   }
 
-  Future<Null> buildOutline(LibraryBuilder library);
+  Future<Null> buildOutline(covariant LibraryBuilder library);
 
   /// Builds all the method bodies found in the given [library].
   ///
   /// [astKind] determines whether or not analyzer ASTs are used as an
   /// intermediate data structure.
-  Future<Null> buildBody(LibraryBuilder library, AstKind astKind);
+  Future<Null> buildBody(covariant LibraryBuilder library, AstKind astKind);
 
   List<InputError> collectCompileTimeErrors() {
     List<InputError> errors = <InputError>[];