Revert "Fix misoptimization of 'is' test"

tests/standalone/regress_29695_test.dart

 // Copyright (c) 2017, 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 type tests are not misoptimized.
 // VMOptions=--optimization-counter-threshold=1000 --optimization-filter=IsAnInt
 
 main() {
   train();
   if (IsAnInt("This is not an int")) throw "oops";
 }
 
 // Prime the IC with things that are and are not ints.
 void train() {
+  int sum = 0;
   for (int i = 0; i < 10000; i++) {
     IsAnInt(42); // Smi - always goes first in the generated code.
     IsAnInt(1 << 62); // Mint on 64 bit platforms.
     IsAnInt(1 << 62);
     IsAnInt(4200000000000000000000000000000000000); // BigInt
     IsAnInt(4200000000000000000000000000000000000);
     // This one that is not an int goes last in the IC because it is called
     // less frequently.
     IsAnInt(4.2);
   }
 }
 
-bool IsAnInt(f) => f is int;
+int IsAnInt(Foo f) => f is int;