All strong mode cleaning of dart2js.

tests/compiler/dart2js/type_combination_test.dart

 // Copyright (c) 2012, 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.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/js_backend/backend.dart' show JavaScriptBackend;
 import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/types/types.dart';
 import 'package:compiler/src/world.dart';
@@ skipping 36 matching lines @@
 TypeMask jsExtendableArrayOrNull;
 TypeMask jsUnmodifiableArrayOrNull;
 TypeMask jsIndexableOrNull;
 TypeMask jsInterceptorOrNull;
 
 class Pair {
   final first;
   final second;
   Pair(this.first, this.second);
   int get hashCode => first.hashCode * 47 + second.hashCode;
-  bool operator ==(Pair other) =>
-      identical(first, other.first) && identical(second, other.second);
+  bool operator ==(other) =>
+      other is Pair &&
+      identical(first, other.first) &&
+      identical(second, other.second);
 }
 
 class RuleSet {
   final name;
   final operate;
   final Set typesSeen = new Set();
   final Set pairsSeen = new Set();
 
   RuleSet(this.name, this.operate);
 
@@ skipping 776 matching lines @@
     Expect.notEquals(jsStringOrNull, potentialString,
         "potentialString expected not to be exact JSString");
     Expect.notEquals(jsArrayOrNull, potentialArray,
         "potentialArray expected not to be JSArray subclass");
 
     testUnion(compiler);
     testIntersection(compiler);
     testRegressions(compiler);
   });
 }