remove most string concat, fixes #7

lib/src/token.dart

 /// This library contains token types used by the html5 tokenizer.
 library token;
 
 import 'dart:collection';
 import 'package:source_span/source_span.dart';
 
 /// An html5 token.
 abstract class Token {
   FileSpan span;
 
@@ skipping 29 matching lines @@
 }
 
 class EndTagToken extends TagToken {
   EndTagToken(String name, {bool selfClosing: false})
       : super(name, selfClosing);
 
   int get kind => TokenKind.endTag;
 }
 
 abstract class StringToken extends Token {
-  String data;
-  StringToken(this.data);
+  StringBuffer _buffer;
+
+  String _string;
+  String get data {
+    if (_string == null) {
+      _string = _buffer.toString();
+      _buffer = null;
+    }
+    return _string;
+  }
+
+  StringToken(string)
+      : _string = string,
+        _buffer = string == null ? new StringBuffer() : null;
+
+  StringToken add(String data) {
+    _buffer.write(data);
+    return this;
+  }
 }
 
 class ParseErrorToken extends StringToken {
   /// Extra information that goes along with the error message.
   Map messageParams;
 
   ParseErrorToken(String data, {this.messageParams}) : super(data);
 
   int get kind => TokenKind.parseError;
 }
 
 class CharactersToken extends StringToken {
   CharactersToken([String data]) : super(data);
 
   int get kind => TokenKind.characters;
+
+  /// Replaces the token's [data]. This should only be used to wholly replace
+  /// data, not to append data.
+  void replaceData(String newData) {
+    _string = newData;
+    _buffer = null;
+  }
 }
 
 class SpaceCharactersToken extends StringToken {
   SpaceCharactersToken([String data]) : super(data);
 
   int get kind => TokenKind.spaceCharacters;
 }
 
 class CommentToken extends StringToken {
   CommentToken([String data]) : super(data);
@@ skipping 19 matching lines @@
   String name;
   String value;
 
   // The spans of the attribute. This is not used unless we are computing an
   // attribute span on demand.
   int start;
   int end;
   int startValue;
   int endValue;
 
-  TagAttribute(this.name, [this.value = '']);
+  TagAttribute();
 }
 
 class TokenKind {
   static const int spaceCharacters = 0;
   static const int characters = 1;
   static const int startTag = 2;
   static const int endTag = 3;
   static const int comment = 4;
   static const int doctype = 5;
   static const int parseError = 6;
 }