Convert raw strings to new syntax

tests/language/metadata_test.dart

 // Copyright (c) 2012, 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 ensuring that compiler can parse metadata. Need to add negative
 // test cases with illegal metadata annotations.
 
 #library("metadata_test.dart");
 #import("metadata_lib.dart", prefix: "Meta");
 
@@ skipping 13 matching lines @@
 @meta1 typedef int DingDong<@meta2 T>(@meta1 event);
 
 @meta1 class A <@Tag("typeParam") T> {
   @meta1 @meta2
   static String staticField;
 
   @Meta.Alien("ET") int foo(@meta1 bool fool, {@meta1 @Tag("opt") x: 100}) {
     return x;
   }
 
-  @Tag(@"timewarp")
+  @Tag(r"timewarp")
   List<int> getNextWeeksLottoNumbers() => [1, 2, 3, 4, 5, 6];
 }
 
 @meta1 main() {
   @meta1 var a = new A();
   Expect.equals(0, a.foo(false, x: 0));
 
   for (@Tag("loopvar") int i = 0; i < 10; i++) {
     // Do something.
   }
 
-  @meta1 var s = @'This is a raw \\ string.';
+  @meta1 var s = r'This is a raw \\ string.';
 }