Setup correct scope for initializers and complain about fields initialized more than once.

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 0e06be6da220a0d88e51f987434bb6d798682cbe..6a1b1449ee583fbac477ad6c9b496b836d55aac4 100644
--- a/pkg/front_end/lib/src/fasta/source/stack_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
@@ -71,10 +71,6 @@ abstract class StackListener extends Listener {
   // and ast_builder.dart.
   void exitLocalScope() => internalError("Unsupported operation");
 
-  // TODO(ahe): This doesn't belong here. Only implemented by body_builder.dart
-  // and ast_builder.dart.
-  void prepareInitializers() => internalError("Unsupported operation");
-
   void push(Object node) {
     if (node == null) internalError("null not allowed.");
     stack.push(node);