Advance html5lib to newest breaking changes in core: getKeys -> keys, etc

test/support.dart

 /** Support code for the tests in this directory. */
 library support;
 
 import 'dart:io';
 import 'package:html5lib/src/treebuilder.dart';
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/dom_parsing.dart';
 
 typedef TreeBuilder TreeBuilderFactory(bool namespaceHTMLElements);
 
@@ skipping 42 matching lines @@
   // it's easier to convert it into an upfront computation.
   Iterator<Map> iterator() => _getData().iterator();
 
   List<Map> _getData() {
     var data = <String, String>{};
     var key = null;
     var result = <Map>[];
     var lines = _text.split('\n');
     int numLines = lines.length;
     // Remove trailing newline to match Python
-    if (lines.last() == '') {
+    if (lines.last == '') {
       lines.removeLast();
     }
     for (var line in lines) {
       var heading = sectionHeading(line);
       if (heading != null) {
         if (data.length > 0 && heading == newTestHeading) {
           // Remove trailing newline
           data[key] = data[key].substring(0, data[key].length - 1);
           result.add(normaliseOutput(data));
           data = <String, String>{};
@@ skipping 81 matching lines @@
     indent += 1;
     for (var child in node.nodes) visit(child);
     indent -= 1;
   }
 
   visitElement(Element node) {
     _newline();
     _str.add(node);
     if (node.attributes.length > 0) {
       indent += 2;
-      var keys = new List.from(node.attributes.getKeys());
+      var keys = new List.from(node.attributes.keys);
       keys.sort((x, y) => x.compareTo(y));
       for (var key in keys) {
         var v = node.attributes[key];
         if (key is AttributeName) {
           AttributeName attr = key;
           key = "${attr.prefix} ${attr.name}";
         }
         _newline();
         _str.add('$key="$v"');
       }
       indent -= 2;
     }
     visitChildren(node);
   }
 }