Fix NPE in parser.

dart/frog/leg/scanner/parser.dart

Index: dart/frog/leg/scanner/parser.dart
diff --git a/dart/frog/leg/scanner/parser.dart b/dart/frog/leg/scanner/parser.dart
index f2427bcb21a2508ed5b0fdb59749ba74d6ad655e..306f082494ecdd69447a564df15a57a84f9cf69d 100644
--- a/dart/frog/leg/scanner/parser.dart
+++ b/dart/frog/leg/scanner/parser.dart
@@ -317,7 +317,14 @@ class Parser {
   Token parseTypeVariablesOpt(Token token) {
     if (optional('<', token)) {
       BeginGroupToken beginGroupToken = token;
-      token = beginGroupToken.endGroup.next;
+      // TODO(ahe): Parse type variables.
+
+      // For now, skip to the matching '>' if it exists. Otherwise,
+      // don't advance and assume the caller will report some kind of
+      // error.
+      if (beginGroupToken.endGroup !== null) {
+        token = beginGroupToken.endGroup.next;
+      }
     }
     listener.handleNoTypeVariables(token);
     return token;