Change our 'is int' check to use Math.floor instead of a smi check.

tests/language/infinity_test.dart

Index: tests/language/infinity_test.dart
===================================================================
--- tests/language/infinity_test.dart	(revision 0)
+++ tests/language/infinity_test.dart	(revision 0)
@@ -0,0 +1,15 @@
+// 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.
+
+int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
+
+main() {
+  var things = [0, double.INFINITY, double.NEGATIVE_INFINITY];
+  var first = things[1];
+  var second = things[2];
+  Expect.isFalse(first is int);
+  Expect.isFalse(second is int);
+  Expect.isTrue(first is double);
+  Expect.isTrue(second is double);
+}