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

lib/compiler/implementation/string_validator.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.
 
 // Check the validity of string literals.
 
 #library("stringvalidator");
 
 #import("leg.dart");
 #import("scanner/scannerlib.dart");
@@ skipping 31 matching lines @@
                           token.charOffset + leftQuote,
                           content,
                           quoting);
   }
 
   static StringQuoting quotingFromString(SourceString sourceString) {
     Iterator<int> source = sourceString.iterator();
     bool raw = false;
     int quoteLength = 1;
     int quoteChar = source.next();
-    if (quoteChar == $AT) {
+    // TODO(aprelev@gmail.com): Remove deprecated "quoteChar === $AT ||" below.
+    if (quoteChar === $AT || quoteChar === $r) {
       raw = true;
       quoteChar = source.next();
     }
     assert(quoteChar === $SQ || quoteChar === $DQ);
     // String has at least one quote. Check it if has three.
     // If it only have two, the string must be an empty string literal,
     // and end after the second quote.
     bool multiline = false;
     if (source.hasNext() && source.next() === quoteChar && source.hasNext()) {
       int code = source.next();
@@ skipping 115 matching lines @@
       }
     }
     // String literal successfully validated.
     if (quoting.raw || !containsEscape) {
       // A string without escapes could just as well have been raw.
       return new DartString.rawString(string, length);
     }
     return new DartString.escapedString(string, length);
   }
 }