Implement initializer asserts.

pkg/front_end/lib/src/fasta/source/stack_listener.dart

Index: pkg/front_end/lib/src/fasta/source/stack_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/stack_listener.dart b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
index ee7ca7b282a77af29d46e2205ef7245bac197338..650c0a18285675373865503a31c5dafffcc5c2ea 100644
--- a/pkg/front_end/lib/src/fasta/source/stack_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
@@ -230,10 +230,15 @@ abstract class StackListener extends Listener {
   }
 
   @override
-  void handleRecoverExpression(Token token) {
+  void handleRecoverExpression(Token token, FastaMessage message) {
     debugEvent("RecoverExpression");
   }
 
+  void handleExtraneousExpression(Token token, FastaMessage message) {
+    debugEvent("ExtraneousExpression");
+    pop(); // Discard the extraneous expression.
+  }
+
   @override
   void endCaseExpression(Token colon) {
     debugEvent("CaseExpression");