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

lib/html/dartium/html_dartium.dart

Index: lib/html/dartium/html_dartium.dart
diff --git a/lib/html/dartium/html_dartium.dart b/lib/html/dartium/html_dartium.dart
index f4a7f67c2d23e0a184a3247d30d6bd6bb13bb0c0..a8498f864789317a99af1ba5b268f82ce5d5997c 100644
--- a/lib/html/dartium/html_dartium.dart
+++ b/lib/html/dartium/html_dartium.dart
@@ -6178,6 +6178,8 @@ class _DocumentImpl extends _NodeImpl implements Document
 
   HeadElement get head() native "Document_head_Getter";
 
+  DOMImplementation get implementation() native "Document_implementation_Getter";
+
   String get lastModified() native "Document_lastModified_Getter";
 
   String get preferredStylesheetSet() native "Document_preferredStylesheetSet_Getter";
@@ -26104,6 +26106,9 @@ interface Document extends HtmlElement {
   /** @domName Document.head */
   final HeadElement head;
 
+  /** @domName Document.implementation */
+  final DOMImplementation implementation;
+
   /** @domName Document.lastModified */
   final String lastModified;