Get rid of two more warnings.

pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.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.kernel_library_builder;
 
 import 'package:kernel/ast.dart';
 
 import 'package:kernel/clone.dart' show
     CloneVisitor;
@@ skipping 329 matching lines @@
   int finishTypeVariables(ClassBuilder object) {
     int count = boundlessTypeVariables.length;
     for (KernelTypeVariableBuilder builder in boundlessTypeVariables) {
       builder.finish(object);
     }
     boundlessTypeVariables.clear();
     return count;
   }
 
   @override
-  void includePart(KernelLibraryBuilder part) {
+  void includePart(covariant KernelLibraryBuilder part) {

[Siggi Cherem (dart-lang), 2017/02/15 22:05:25]

happy to do it differently here, since the other covariant are still in use, we
can change them all at once.

     super.includePart(part);
     nativeMethods.addAll(part.nativeMethods);
     boundlessTypeVariables.addAll(part.boundlessTypeVariables);
     mixinApplicationClasses.addAll(part.mixinApplicationClasses);
   }
 }
 
 bool isConstructorName(String name, String className) {
   if (name.startsWith(className)) {
     if (name.length == className.length) return true;
     if (name.startsWith(".", className.length)) return true;
   }
   return false;
 }