Reduce strong mode errors and warnings

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.loader;
 
 import 'dart:async' show
     Future;
 
 import 'dart:collection' show
@@ skipping 112 matching lines @@
       // TODO(ahe): Share this message with [buildBodies]. Also make it easy to
       // tell the difference between outlines read from a dill file or source
       // files. Currently, [libraryCount] is wrong for dill files.
       print("""
 $sinceStart: $message ($byteCount bytes) in ${format(ms, 3, 0)}ms, that is,
 ${format(byteCount / ms, 3, 12)} bytes/ms, and
 ${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>[];
     for (LibraryBuilder library in builders.values) {
       if (library.loader == this) {
         errors.addAll(library.compileTimeErrors);
       }
     }
     return errors;
   }
 }
 
 String format(double d, int fractionDigits, int width) {
   return d.toStringAsFixed(fractionDigits).padLeft(width);
 }