Changed UnsupportedOperationException to UnsupportedError

lib/src/token.dart

 /** This library contains token types used by the html5 tokenizer. */
 library token;
 
 import 'package:html5lib/dom_parsing.dart' show SourceSpan;
 
 /** An html5 token. */
 abstract class Token {
   SourceSpan span;
 
   abstract int get kind;
@@ skipping 74 matching lines @@
   String systemId;
   String name;
   bool correct;
 
   DoctypeToken({this.publicId, this.systemId, this.correct: false})
       : name = "";
 
   int get kind => TokenKind.doctype;
 
   // TODO(jmesserly): remove. These are only here because of Token.data
-  String get data { throw const UnsupportedOperationException("data"); }
-  set data(value) { throw const UnsupportedOperationException("data"); }
+  String get data { throw new UnsupportedError("data"); }
+  set data(value) { throw new UnsupportedError("data"); }
 }
 
 
 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;
 }