Add special rules in the inferrer that some int operations return an int.

tests/compiler/dart2js/mock_compiler.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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 import 'dart:uri';
 
@@ skipping 94 matching lines @@
     operator >>(other) => 42;
     operator |(other) => 42;
     operator &(other) => 42;
     operator ^(other) => 42;
 
     operator >(other) => true;
     operator >=(other) => true;
     operator <(other) => true;
     operator <=(other) => true;
     operator ==(other) => true;
+
+    abs() => (this is JSInt) ? 42 : 42.0;
+    remainder(other) => (this is JSInt) ? 42 : 42.0;
   }
   class JSInt extends JSNumber implements int {
   }
   class JSDouble extends JSNumber implements double {
   }
   class JSNull extends Interceptor {
     bool operator==(other) => identical(null, other);
   }
   class JSBool extends Interceptor implements bool {
   }
@@ skipping 269 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }