Making Document, Element and Window documentable via @DocsEditable

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 97 matching lines @@
           "   *",
           "   * The [type] can have values such as:",
           "   *",
           "   * * `'Text'`",
           "   * * `'URL'`",
           "   */"
         ]
       }
     },
     "Document": {
+      "comment": [
+        "/**",
+        " * The base class for all documents.",
+        " *",
+        " * Each web page loaded in the browser has its own [Document] object, which is",
+        " * typically an [HtmlDocument].",
+        " *",
+        " * If you aren't comfortable with DOM concepts, see the Dart tutorial",
+        " * [Target 2: Connect Dart & HTML](http://www.dartlang.org/docs/tutorials/connect-dart-html/).",
+        " */"
+      ],
       "members": {
         "createElement": [
           "/// Deprecated: use new Element.tag(tagName) instead."
         ],
         "querySelector": [
           "/**",
           " * Finds the first descendant element of this document that matches the",
           " * specified group of selectors.",
           " *",
           " * Unless your webpage contains multiple documents, the top-level query",
           " * method behaves the same as this method, so you should use it instead to",
           " * save typing a few characters.",
           " *",
           " * [selectors] should be a string using CSS selector syntax.",
           " *     var element1 = document.query('.className');",
           " *     var element2 = document.query('#id');",
           " *",
           " * For details about CSS selector syntax, see the",
           " * [CSS selector specification](http://www.w3.org/TR/css3-selectors/).",
           " */"
         ]
       }
     },
     "Element": {
+      "comment": [
+        "/**",
+        " * An abstract class, which all HTML elements extend.",
+        " */"
+      ],
       "members": {
         "querySelector": [
           "/**",
           " * Finds the first descendant element of this element that matches the",
           " * specified group of selectors.",
           " *",
           " * [selectors] should be a string using CSS selector syntax.",
           " *",
           " *     // Gets the first descendant with the class 'classname'",
           " *     var element = element.query('.className');",
@@ skipping 367 matching lines @@
           "   *",
           "   * [oldVersion] should match the database's current [version] exactly.",
           "   *",
           "   * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.",
           "   */"
         ]
       }
     }
   }
 }