Only emit typeguards if we think they are valuable.

frog/tests/leg/src/TypeGuardUnuserTest.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(a) {
   int c = foo(1);
   if (a) c = foo(2);
@@ skipping 11 matching lines @@
   return a + c;
 }
 """;
 
 final String TEST_THREE = @"""
 foo(int param1, int param2) {
   return 0 + param1 + param2;
 }
 """;
 
+final String TEST_THREE_WITH_BAILOUT = @"""
+foo(int param1, int param2) {
+  var t;
+  for (int i = 0; i < 1; i++) {
+    t = 0 + param1 + param2;
+  }
+  return t;
+}
+""";
 
 main() {
   String generated = compile(TEST_ONE, 'foo');
   RegExp regexp = new RegExp(getIntTypeCheck(anyIdentifier));
   Iterator<Match> matches = regexp.allMatches(generated).iterator();
   checkNumberOfMatches(matches, 0);
 
   regexp = const RegExp("return c;");
   Expect.isTrue(regexp.hasMatch(generated));
 
   generated = compile(TEST_TWO, 'foo');
   regexp = const RegExp("foo\\(1\\)");
   matches = regexp.allMatches(generated).iterator();
   checkNumberOfMatches(matches, 1);
 
   generated = compile(TEST_THREE, 'foo');
   regexp = new RegExp(getNumberTypeCheck('param1'));
+  Expect.isTrue(!regexp.hasMatch(generated));
+  regexp = new RegExp(getNumberTypeCheck('param2'));
+  Expect.isTrue(!regexp.hasMatch(generated));
+
+  generated = compile(TEST_THREE_WITH_BAILOUT, 'foo');
+  regexp = new RegExp(getNumberTypeCheck('param1'));
   Expect.isTrue(regexp.hasMatch(generated));
   regexp = new RegExp(getNumberTypeCheck('param2'));
   Expect.isTrue(regexp.hasMatch(generated));
 }