issue 4817 fixed. Implemented raw strings with 'r' instead of '@' for dart2js

tests/compiler/dart2js_native/native_named_constructors3_frog_test.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.
 
 // Hidden native class with factory constructors and NO static methods.
 // Regression test.
 
 
 class A native "*A" {
 
   // No static methods in this class.
 
   factory A(int len) => makeA(len);
 
   factory A.fromString(String s)  => makeA(s.length);
 
-  factory A.nativeConstructor(int a, int b) native @'return makeA(a+b);';
+  factory A.nativeConstructor(int a, int b) native r'return makeA(a+b);';
 
   foo() native 'return this._x;';
 }
 
 makeA(v) native;
 
 void setup() native """
 // This code is all inside 'setup' and so not accesible from the global scope.
 function A(arg) { this._x = arg; }
 makeA = function(arg) { return new A(arg); }
 """;
 
 main() {
   setup();
   var a1 = new A(100);
   var a2 = new A.fromString('Hello');
   var a3 = new A.nativeConstructor(100, 2);
 
   Expect.equals(100, a1.foo());
   Expect.equals(5, a2.foo());
   Expect.equals(102, a3.foo());
 }