Roll Observatory packages and add a roll script

packages/html/lib/dom.dart

 /// A simple tree API that results from parsing html. Intended to be compatible
 /// with dart:html, but it is missing many types and APIs.
 library dom;
 
 // TODO(jmesserly): lots to do here. Originally I wanted to generate this using
 // our Blink IDL generator, but another idea is to directly use the excellent
 // http://dom.spec.whatwg.org/ and http://html.spec.whatwg.org/ and just
 // implement that.
 
 import 'dart:collection';
@@ skipping 984 matching lines @@
 
 class _ConcatTextVisitor extends TreeVisitor {
   final _str = new StringBuffer();
 
   String toString() => _str.toString();
 
   visitText(Text node) {
     _str.write(node.data);
   }
 }