Make sure we're not trying to optimize an int check if the input can be a number.

tests/compiler/dart2js_extra/inferrer_is_int_test.dart

+// Copyright (c) 2013, 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.
+
+// Test that dart2js's inferrer and code optimizers know a double
+// literal might become an int at runtime.
+
+import "package:expect/expect.dart";
+import '../../language/compiler_annotations.dart';
+
+@DontInline()
+callWithStringAndDouble(value) {
+  () => 42;
+  if (value is !int) throw new ArgumentError(value);
+  return 42;
+}
+
+@DontInline()
+callWithDouble(value) {
+  () => 42;
+  if (value is !int) throw new ArgumentError(value);
+  return 42;
+}
+
+main() {
+  Expect.throws(() => callWithStringAndDouble('foo'),
+                (e) => e is ArgumentError);
+  Expect.equals(42, callWithStringAndDouble(0.0));
+  Expect.equals(42, callWithDouble(0.0));
+}