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

tests/compiler/dart2js_foreign/native_named_constructors3_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.
 
 
 @native("*A")
 class A  {
 
   // No static methods in this class.
 
   factory A(int len) => makeA(len);
 
   factory A.fromString(String s)  => makeA(s.length);
 
-  @native(@'return makeA(a+b);')
+  @native(r'return makeA(a+b);')
   factory A.nativeConstructor(int a, int b);
 
   @native('return this._x;')
   foo();
 }
 
 @native makeA(v);
 
 @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); }
 """)
 void setup();
 
 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());
 }