Convert raw strings to new syntax

tests/language/field_method4_negative_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.
 // Dart test to catch error reporting bugs when using a field like a method.
 
 class A {
   var foo;
   A() {
     foo = () { };
   }
   void bar(var a) {
     a.foo();  // Tries to invoke the non-existing method 'foo'.
     /*
     'a.foo()' is a "Regular instance-method invocation". The guide says:
     "If no method is found, the result of the invocation expression is
-    equivalent to: $0.noSuchMethod(@"id", [$1, ..., $N])."
+    equivalent to: $0.noSuchMethod(r"id", [$1, ..., $N])."
     Invoking noSuchMethod on an instance of A will invoke Object's
     noSuchMethod (because A doesn't override that method). Object's
     noSuchMethod will throw an error.
     */
   }
 }
 
 class FieldMethod4NegativeTest {
   static testMain() {
     var a = new A();
     a.bar();
   }
 }
 
 
 main() {
   FieldMethod4NegativeTest.testMain();
 }