Require two type arguments for map literals (issue 4522).

samples/markdown/lib.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.
 
 /// Parses text in markdown format. Use this entrypoint if you want to parse
 /// markdown from your own Dart code. To parse markdown by running the script
 /// directly from the command line, see markdown.dart.
 #library('markdown');
 
 #source('ast.dart');
@@ skipping 22 matching lines @@
 
 Node setImplicitLinkResolver(Node resolver(String text)) {
   _implicitLinkResolver = resolver;
 }
 
 /// Maintains the context needed to parse a markdown document.
 class Document {
   final Map<String, Link> refLinks;
 
   Document()
-    : refLinks = <Link>{};
+    : 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'.
@@ skipping 53 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);
 }