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

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.scope;
 
 import 'builder.dart' show Builder, MixedAccessor;
 
 import '../errors.dart' show internalError;
 
@@ skipping 90 matching lines @@
     }
   }
 
   void forwardDeclareLabel(String name, Builder target) {
     declareLabel(name, target);
     forwardDeclaredLabels ??= <String, Builder>{};
     forwardDeclaredLabels[name] = target;
   }
 
   void claimLabel(String name) {
+    if (forwardDeclaredLabels == null) return;
     forwardDeclaredLabels.remove(name);
     if (forwardDeclaredLabels.length == 0) {
       forwardDeclaredLabels = null;
     }
   }
 
   Map<String, Builder> get unclaimedForwardDeclarations {
     return forwardDeclaredLabels;
   }
 
@@ skipping 47 matching lines @@
 class AmbiguousBuilder extends Builder {
   final Builder builder;
 
   AmbiguousBuilder(this.builder, int charOffset, Uri fileUri)
       : super(null, charOffset, fileUri);
 
   get target => null;
 
   bool get hasProblem => true;
 }