Fix more strong mode errors in analyzer

pkg/analyzer/lib/src/generated/type_system.dart

 // Copyright (c) 2015, 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 analyzer.src.generated.type_system;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 855 matching lines @@
   // TODO(jmesserly): this is very similar to code used for flattening futures.
   // The only difference is, because of a lack of TypeProvider, the other method
   // has to match the Future type by its name and library. Here was are passed
   // in the correct type.
   DartType mostSpecificTypeArgument(DartType type, DartType genericType) {
     if (type is! InterfaceType) return null;
 
     // Walk the superinterface hierarchy looking for [genericType].
     List<DartType> candidates = <DartType>[];
     HashSet<ClassElement> visitedClasses = new HashSet<ClassElement>();
-    void recurse(InterfaceTypeImpl interface) {
+    void recurse(InterfaceType interface) {
       if (interface.element == genericType.element &&
           interface.typeArguments.isNotEmpty) {
         candidates.add(interface.typeArguments[0]);
       }
       if (visitedClasses.add(interface.element)) {
         if (interface.superclass != null) {
           recurse(interface.superclass);
         }
         interface.mixins.forEach(recurse);
         interface.interfaces.forEach(recurse);
@@ skipping 410 matching lines @@
       } else {
         passedOut = true;
       }
     } else if (type is FunctionType) {
       _visitFunctionType(typeParam, type, paramIn);
     } else if (type is InterfaceType) {
       _visitInterfaceType(typeParam, type, paramIn);
     }
   }
 }