Roll Observatory packages and add a roll script

packages/html/lib/dom_parsing.dart

 /// This library contains extra APIs that aren't in the DOM, but are useful
 /// when interacting with the parse tree.
 library dom_parsing;
 
 import 'dom.dart';
 import 'src/constants.dart' show rcdataElements;
 
 /// A simple tree visitor for the DOM nodes.
 class TreeVisitor {
   visit(Node node) {
@@ skipping 183 matching lines @@
   final parent = node.parentNode;
   if (parent is Element) {
     var tag = parent.localName;
     if (rcdataElements.contains(tag) || tag == 'plaintext') {
       str.write(node.data);
       return;
     }
   }
   str.write(htmlSerializeEscape(node.data));
 }