Changed UnsupportedOperationException to UnsupportedError

lib/src/token.dart

Index: lib/src/token.dart
diff --git a/lib/src/token.dart b/lib/src/token.dart
index f95a6462d85175a1da7a56bb0dc2e11d97ed6488..7fdf13ca8fbc386c0933bf34830049b47e1525e8 100644
--- a/lib/src/token.dart
+++ b/lib/src/token.dart
@@ -92,8 +92,8 @@ class DoctypeToken extends Token {
   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"); }
 }