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

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
===================================================================
--- tests/compiler/dart2js/mock_compiler.dart	(revision 21800)
+++ tests/compiler/dart2js/mock_compiler.dart	(working copy)
@@ -112,6 +112,9 @@
     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 {
   }