Fix some strong mode issues.

lib/type_algebra.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.type_algebra;
 
 import 'ast.dart';
 
 /// Returns a type where all occurrences of the given type parameters have been
 /// replaced with the corresponding types.
 ///
@@ skipping 344 matching lines @@
   int depth = 0;
   bool isInfinite = false;
 
   _DeepTypeSubstitutor(Map<TypeParameter, DartType> substitution,
       [_DeepTypeSubstitutor outer])
       : super(outer) {
     this.substitution.addAll(substitution);
   }
 
   @override
-  _TypeSubstitutor newInnerEnvironment() {
+  _DeepTypeSubstitutor newInnerEnvironment() {
     return new _DeepTypeSubstitutor(<TypeParameter, DartType>{}, this);
   }
 
   @override
   DartType visitTypeParameterType(TypeParameterType node) {
     DartType replacement = getSubstitute(node.parameter);
     if (replacement == null) return node;
     if (isInfinite) return replacement;
     ++depth;
     if (depth > substitution.length) {
@@ skipping 200 matching lines @@
 }
 
 Map<dynamic/*=K*/, dynamic/*=W*/ > _mapValues/*<K,V,W>*/(
     Map<dynamic/*=K*/, dynamic/*=V*/ > map, dynamic/*=W*/ fn(dynamic/*=V*/)) {
   Map<dynamic/*=K*/, dynamic/*=W*/ > result = {};
   map.forEach((key, value) {
     result[key] = fn(value);
   });
   return result;
 }