remove most string concat, fixes #7

lib/src/treebuilder.dart

 /// Internals to the tree builders.
 library treebuilder;
 
 import 'dart:collection';
 import 'package:html/dom.dart';
 import 'package:html/parser.dart' show getElementNameTuple;
 import 'package:source_span/source_span.dart';
 import 'constants.dart';
 import 'list_proxy.dart';
 import 'token.dart';
@@ skipping 306 matching lines @@
   }
 
   /// Insert [data] as text in the current node, positioned before the
   /// start of node [refNode] or to the end of the node's text.
   static void _insertText(Node parent, String data, FileSpan span,
       [Element refNode]) {
     var nodes = parent.nodes;
     if (refNode == null) {
       if (nodes.length > 0 && nodes.last is Text) {
         Text last = nodes.last;
-        last.data = '${last.data}$data';
+        last.appendData(data);
 
         if (span != null) {
           last.sourceSpan =
               span.file.span(last.sourceSpan.start.offset, span.end.offset);
         }
       } else {
         nodes.add(new Text(data)..sourceSpan = span);
       }
     } else {
       int index = nodes.indexOf(refNode);
       if (index > 0 && nodes[index - 1] is Text) {
         Text last = nodes[index - 1];
-        last.data = '${last.data}$data';
+        last.appendData(data);
       } else {
         nodes.insert(index, new Text(data)..sourceSpan = span);
       }
     }
   }
 
   /// Get the foster parent element, and sibling to insert before
   /// (or null) when inserting a misnested table node
   List<Node> getTableMisnestedNodePosition() {
     // The foster parent element is the one which comes before the most
@@ skipping 48 matching lines @@
   Document getDocument() => document;
 
   /// Return the final fragment.
   DocumentFragment getFragment() {
     //XXX assert innerHTML
     var fragment = new DocumentFragment();
     openElements[0].reparentChildren(fragment);
     return fragment;
   }
 }