Two minor bug fixes: a type error in the parser, and throwing the

dart/frog/leg/scanner/partial_parser.dart

Index: dart/frog/leg/scanner/partial_parser.dart
diff --git a/dart/frog/leg/scanner/partial_parser.dart b/dart/frog/leg/scanner/partial_parser.dart
index 98a4b6946b70b7b3c797364c5cd9ebe86c38196a..5671681629c1060870599c0faa48478e4079cb41 100644
--- a/dart/frog/leg/scanner/partial_parser.dart
+++ b/dart/frog/leg/scanner/partial_parser.dart
@@ -49,10 +49,17 @@ class PartialParser extends Parser {
 
   Token parseFormalParameters(Token token) => skipFormals(token);
 
-  Token skipFormals(BeginGroupToken token) {
+  Token skipFormals(Token token) {
     listener.beginOptionalFormalParameters(token);
-    expect('(', token);
-    Token endToken = token.endGroup;
+    if (!optional('(', token)) {
+      if (optional(';', token)) {
+        listener.recoverableError("expected '('", token: token);
+        return token;
+      }
+      return listener.unexpected(token);
+    }
+    BeginGroupToken beginGroupToken = token;
+    Token endToken = beginGroupToken.endGroup;
     listener.endFormalParameters(0, token, endToken);
     return endToken.next;
   }