Added Area Element class doc.

tools/dom/docs/docs.json

 {
   "dart.dom.html": {
     "CanvasGradient": {
       "comment": [
         "/**",
         " * An opaque canvas object representing a gradient.",
         " *",
         " * Created by calling [createLinearGradient] or [createRadialGradient] on a",
         " * [CanvasRenderingContext2D] object.",
         " *",
@@ skipping 141 matching lines @@
           "/// Moved to [HtmlDocument]."
         ],
         "webkitIsFullScreen": [
           "/// Moved to [HtmlDocument]."
         ],
         "webkitPointerLockElement": [
           "/// Moved to [HtmlDocument]."
         ]
       }
     },
+    "HTMLAreaElement": {
+      "comment": [
+        "/**",
+        " * DOM Area Element, which links regions of an image map with a hyperlink.",
+        " *",
+        " * The element can also define an uninteractive region of the map.",
+        " *",
+        " * See also:",
+        " *",
+        " * * [Area Element](https://developer.mozilla.org/en-US/docs/HTML/Element/area)",
+        " * from MDN.",
+        " */"
+      ]
+    },
     "HTMLCanvasElement": {
       "members": {
         "height": [
           "/// The height of this canvas element in CSS pixels."
         ],
         "toDataURL": [
           "/**",
           "   * Returns a data URI containing a representation of the image in the",
           "   * format specified by type (defaults to 'image/png').",
           "   *",
@@ skipping 233 matching lines @@
           "   * is the (more verbose) equivalent of",
           "   *",
           "   *     var request = new HttpRequest.get('http://dartlang.org',",
           "   *         (event) => print('Request complete'));",
           "   */"
         ]
       }
     }
   }
 }