fix issues link references and prototypes

Link with names that clashed with properties inherited from the
Object prototype (such as "constructor") were not expanding. This fixes
this issue.

Before this change, markdown of this form...:

    Link: [constructor][].

    [constructor]: https://example.org/

...resulted in HTML output of this form:

    <p>Link: [constructor][].</p>

With this change, it now renders as expected:

    <p>Link: <a href="https://example.org/">constructor</a>.</p>

lib/marked.js

@@ -135,7 +135,7 @@ block.pedantic = merge({}, block.normal, {
 
 function Lexer(options) {
   this.tokens = [];
-  this.tokens.links = {};
+  this.tokens.links = Object.create(null);
   this.options = options || marked.defaults;
   this.rules = block.normal;
 

test/specs/marked/marked-spec.js

@@ -47,6 +47,20 @@ describe('Marked Code spans', function() {
   });
 });
 
+describe('Marked Links', function() {
+  var section = 'Links';
+
+  var shouldPassButFails = [];
+
+  var willNotBeAttemptedByCoreTeam = [];
+
+  var ignore = shouldPassButFails.concat(willNotBeAttemptedByCoreTeam);
+
+  markedSpec.forEach(function(spec) {
+    messenger.test(spec, section, ignore);
+  });
+});
+
 describe('Marked Table cells', function() {
   var section = 'Table cells';
 

test/specs/marked/marked.json

@@ -52,5 +52,11 @@
     "markdown": "|1|2|\n|-|-|\n| |2|",
     "html": "<table><thead><tr><th>1</th><th>2</th></tr></thead><tbody><tr><td></td><td>2</td></tr></tbody></table>",
     "example": 9
+  },
+  {
+    "section": "Links",
+    "markdown": "Link: [constructor][].\n\n[constructor]: https://example.org/",
+    "html": "<p>Link: <a href=\"https://example.org/\">constructor</a>.</p>",
+    "example": 10
   }
 ]