Fix some strong mode issues.

test/class_hierarchy_basic.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 kernel.class_hierarchy_basic;
 
-import 'package:kernel/class_hierarchy.dart';
 import 'package:kernel/type_algebra.dart';
 import 'package:kernel/ast.dart';
 
 /// A simple implementation of the class hierarchy interface using
 /// hash tables for everything.
-class BasicClassHierarchy implements ClassHierarchy {
+class BasicClassHierarchy {
   final Map<Class, Set<Class>> superclasses = <Class, Set<Class>>{};
   final Map<Class, Set<Class>> superMixtures = <Class, Set<Class>>{};
   final Map<Class, Set<Class>> supertypes = <Class, Set<Class>>{};
   final Map<Class, Map<Class, Supertype>> supertypeInstantiations =
       <Class, Map<Class, Supertype>>{};
   final Map<Class, Map<Name, Member>> gettersAndCalls =
       <Class, Map<Name, Member>>{};
   final Map<Class, Map<Name, Member>> setters = <Class, Map<Name, Member>>{};
   final Map<Class, Map<Name, List<Member>>> interfaceGettersAndCalls =
       <Class, Map<Name, List<Member>>>{};
@@ skipping 221 matching lines @@
   int getClassIndex(Class node) {
     return classIndex[node];
   }
 
   List<int> getExpenseHistogram() => <int>[];
   double getCompressionRatio() => 0.0;
   int getSuperTypeHashTableSize() => 0;
 
   noSuchMethod(inv) => super.noSuchMethod(inv);
 }