Reapply "Refactor constant part." (r4958) with fixes.

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) {
   return a.length;
 }
 """;
 
 final String TEST_TWO = @"""
 foo() {
   return "foo".length;
 }
 """;
 
 final String TEST_THREE = @"""
 foo() {
   return @"foo".length;
 }
 """;
 
 main() {
   String generated = compile(TEST_ONE, 'foo');
   Expect.isTrue(generated.contains("return a.length;"));
 
   generated = compile(TEST_TWO, 'foo');
-  Expect.isTrue(generated.contains("return 3;"));
+  Expect.isTrue(generated.contains("return (3);"));
 
   generated = compile(TEST_THREE, 'foo');
-  Expect.isTrue(generated.contains("return 3;"));
+  Expect.isTrue(generated.contains("return (3);"));
 }