Refactoring of string literals. Implement static string addition.

frog/tests/leg/src/ConstantFoldingTest.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.
 // Test constant folding on numbers.
 
 #import("compiler_helper.dart");
 
 final String NUMBER_FOLDING = """
 void main() {
   var a = 4;
@@ skipping 34 matching lines @@
   String generated = compile(NULL_EQUALS_FOLDING, 'foo');
   RegExp regexp = const RegExp(@'eqNull\$1\(a\)');
   Expect.isTrue(regexp.hasMatch(generated));
 
   regexp = const RegExp(@'\(void 0\) === b');
   Expect.isTrue(regexp.hasMatch(generated));
 
   regexp = const RegExp('4 === c');
   Expect.isTrue(regexp.hasMatch(generated));
 
-  regexp = const RegExp('"foo" === d');
+  regexp = const RegExp("'foo' === d");
   Expect.isTrue(regexp.hasMatch(generated));
 }