Implement fall-through errors.

dart_vm_standalone/rasta_errors.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.md file.
 library dart.core._rasta_errors;
 
 // RastaK generates calls to these methods -- all backends must provide them
 // in their patch for "dart:core".
 //
 // In the future, we could have a single `rasta_errors.dart` for all backends to
 // ensure consistent error messages across all backends.
@@ skipping 166 matching lines @@
     List arguments,
     Map<Symbol, dynamic> namedArguments,
     List existingArgumentNames) {
   return new NoSuchMethodError(receiver, methodName, arguments, namedArguments,
       existingArgumentNames);
 }
 
 _malformedTypeError(String errorMessage) {
   return new _TypeError._create(null, null, null, errorMessage);
 }
+
+_fallThroughError() {
+  return new FallThroughError();
+}