Only emit typeguards if we think they are valuable.

frog/tests/leg/src/BuiltinInterceptorTest.dart

 // Copyright (c) 2011, 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.
 
 #import("compiler_helper.dart");
 
 final String TEST_ONE = @"""
 foo(String a) {
-  // index into the parameter to make sure we'll get a type guard.
-  print(a[0]);
+  // index into the parameter and move into a loop to make sure we'll get a
+  // type guard.
+  for (int i = 0; i < 1; i++) {
+    print(a[0]);
+  }
   return a.length;
 }
 """;
 
 final String TEST_TWO = @"""
 foo() {
   return "foo".length;
 }
 """;
 
@@ skipping 15 matching lines @@
 
   generated = compile(TEST_TWO, 'foo');
   Expect.isTrue(generated.contains("return 3;"));
 
   generated = compile(TEST_THREE, 'foo');
   Expect.isTrue(generated.contains("return 3;"));
 
   generated = compile(TEST_FOUR, 'foo');
   Expect.isTrue(generated.contains("push(2);"));
 }