migrate raw string to new syntax in dartdoc

pkg/dartdoc/lib/markdown.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.
 
 /// Parses text in a markdown-like format and renders to HTML.
 #library('markdown');
 
 // TODO(rnystrom): Use "package:" URL (#4968).
 #source('src/markdown/ast.dart');
 #source('src/markdown/block_parser.dart');
@@ skipping 28 matching lines @@
   final Map<String, Link> refLinks;
 
   Document()
     : refLinks = <String, Link>{};
 
   parseRefLinks(List<String> lines) {
     // This is a hideous regex. It matches:
     // [id]: http:foo.com "some title"
     // Where there may whitespace in there, and where the title may be in
     // single quotes, double quotes, or parentheses.
-    final indent = @'^[ ]{0,3}'; // Leading indentation.
-    final id = @'\[([^\]]+)\]';  // Reference id in [brackets].
-    final quote = @'"[^"]+"';    // Title in "double quotes".
-    final apos = @"'[^']+'";     // Title in 'single quotes'.
-    final paren = @"\([^)]+\)";  // Title in (parentheses).
+    final indent = r'^[ ]{0,3}'; // Leading indentation.
+    final id = r'\[([^\]]+)\]';  // Reference id in [brackets].
+    final quote = r'"[^"]+"';    // Title in "double quotes".
+    final apos = r"'[^']+'";     // Title in 'single quotes'.
+    final paren = r"\([^)]+\)";  // Title in (parentheses).
     final pattern = new RegExp(
         '$indent$id:\\s+(\\S+)\\s*($quote|$apos|$paren|)\\s*\$');
 
     for (int i = 0; i < lines.length; i++) {
       final match = pattern.firstMatch(lines[i]);
       if (match != null) {
         // Parse the link.
         var id = match[1];
         var url = match[2];
         var title = match[3];
@@ skipping 42 matching lines @@
   /// `<em>this <strong>is</strong> a</em> <code>markdown</code>`.
   List<Node> parseInline(String text) => new InlineParser(text, this).parse();
 }
 
 class Link {
   final String id;
   final String url;
   final String title;
   Link(this.id, this.url, this.title);
 }