remove most string concat, fixes flutter#7

This fixes most of the egregious cases of string concat in the HTML tokenizer:
* attribute names/values
* data/comments
* script/rawtext/rcdata

It also fixes adjacent text nodes in TreeBuilder, which happens whenever space characters are adjacent to characters.

There's still an issue of preserving char codes for longer, but this should get out of the O(N^2) at least.

R=sigmund@google.com

Review URL: https://codereview.chromium.org//987433005

Author: John Messerly

lib/dom.dart

@@ -402,18 +402,32 @@ class DocumentType extends Node {
 }
 
 class Text extends Node {
-  String data;
+  /// The text node's data, stored as either a String or StringBuffer.
+  /// We support storing a StringBuffer here to support fast [appendData].
+  /// It will flatten back to a String on read.
+  var _data;
 
-  Text(this.data) : super._();
+  Text(String data) : _data = data != null ? data : '', super._();
 
   int get nodeType => Node.TEXT_NODE;
 
+  String get data => _data = _data.toString();
+  set data(String value) {
+    _data = value != null ? value : '';
+  }
+
   String toString() => '"$data"';
 
   void _addOuterHtml(StringBuffer str) => writeTextNodeAsHtml(str, this);
 
   Text clone(bool deep) => new Text(data);
 
+  void appendData(String data) {
+    if (_data is! StringBuffer) _data = new StringBuffer(_data);
+    StringBuffer sb = _data;
+    sb.write(data);
+  }
+
   String get text => data;
   set text(String value) {
     data = value;
@@ -539,13 +553,19 @@ class Element extends Node with _ParentNode, _ElementAndDocument {
   void _addOuterHtml(StringBuffer str) {
     // http://www.whatwg.org/specs/web-apps/current-work/multipage/the-end.html#serializing-html-fragments
     // Element is the most complicated one.
-    str.write('<${_getSerializationPrefix(namespaceUri)}$localName');
+    str.write('<');
+    str.write(_getSerializationPrefix(namespaceUri));
+    str.write(localName);
 
     if (attributes.length > 0) {
       attributes.forEach((key, v) {
         // Note: AttributeName.toString handles serialization of attribute
         // namespace, if needed.
-        str.write(' $key="${htmlSerializeEscape(v, attributeMode: true)}"');
+        str.write(' ');
+        str.write(key);
+        str.write('="');
+        str.write(htmlSerializeEscape(v, attributeMode: true));
+        str.write('"');
       });
     }
 

lib/parser.dart

@@ -3395,7 +3395,7 @@ class InForeignContentPhase extends Phase {
 
   Token processCharacters(CharactersToken token) {
     if (token.data == "\u0000") {
-      token.data = "\uFFFD";
+      token.replaceData("\uFFFD");
     } else if (parser.framesetOK && !allWhitespace(token.data)) {
       parser.framesetOK = false;
     }

lib/src/token.dart

@@ -47,8 +47,25 @@ class EndTagToken extends TagToken {
 }
 
 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 {
@@ -64,6 +81,13 @@ 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 {
@@ -103,7 +127,7 @@ class TagAttribute {
   int startValue;
   int endValue;
 
-  TagAttribute(this.name, [this.value = '']);
+  TagAttribute();
 }
 
 class TokenKind {