Don't crash on non-forward labels in switches.

pkg/front_end/lib/src/fasta/builder/scope.dart

Index: pkg/front_end/lib/src/fasta/builder/scope.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/scope.dart b/pkg/front_end/lib/src/fasta/builder/scope.dart
index 10e5c39c864c2817993f03e92f5700b05b310062..60e4f9df796c23a62401fa05c64f9f64ca8276a1 100644
--- a/pkg/front_end/lib/src/fasta/builder/scope.dart
+++ b/pkg/front_end/lib/src/fasta/builder/scope.dart
@@ -108,6 +108,7 @@ class Scope {
   }
 
   void claimLabel(String name) {
+    if (forwardDeclaredLabels == null) return;
     forwardDeclaredLabels.remove(name);
     if (forwardDeclaredLabels.length == 0) {
       forwardDeclaredLabels = null;