Tighten type of 'document' to HtmlDocument

tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
+
+$if DART2JS
+  @DomName('Document.body')
+  BodyElement body;
+$else
   @DomName('Document.body')
   BodyElement get body => $dom_body;
 
   @DomName('Document.body')
   void set body(BodyElement value) {
     $dom_body = value;
   }
+$endif
 
   @DomName('Document.caretRangeFromPoint')
   Range caretRangeFromPoint(int x, int y) {
     return $dom_caretRangeFromPoint(x, y);
   }
 
   @DomName('Document.elementFromPoint')
   Element elementFromPoint(int x, int y) {
     return $dom_elementFromPoint(x, y);
   }
@@ skipping 125 matching lines @@
   @Experimental
   Element get pointerLockElement =>
       $dom_webkitPointerLockElement;
 
   @DomName('Document.webkitVisibilityState')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
   @Experimental
   String get visibilityState => $dom_webkitVisibilityState;
 }