Enable document.implementation as it is useful when implementing a web components polyfill among ot…

lib/html/dart2js/html_dart2js.dart

Index: lib/html/dart2js/html_dart2js.dart
diff --git a/lib/html/dart2js/html_dart2js.dart b/lib/html/dart2js/html_dart2js.dart
index 7fdef38f82da949c744a28d7ef692389a19d1b77..3e98282a317842b437db9b1dc119c0b3c51f83c1 100644
--- a/lib/html/dart2js/html_dart2js.dart
+++ b/lib/html/dart2js/html_dart2js.dart
@@ -4717,6 +4717,8 @@ class _DocumentImpl extends _NodeImpl implements Document
 
   final _HeadElementImpl head;
 
+  final _DOMImplementationImpl implementation;
+
   final String lastModified;
 
   final String preferredStylesheetSet;
@@ -22744,6 +22746,9 @@ interface Document extends HtmlElement {
   /** @domName Document.head */
   final HeadElement head;
 
+  /** @domName Document.implementation */
+  final DOMImplementation implementation;
+
   /** @domName Document.lastModified */
   final String lastModified;