Recover correctly from empty optional parameters.

pkg/front_end/lib/src/fasta/kernel/kernel_target.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.kernel_target;
 
 import 'dart:async' show Future;
 
 import 'dart:io' show File, IOSink;
 
@@ skipping 50 matching lines @@
 
 import '../dill/dill_target.dart' show DillTarget;
 
 import '../errors.dart'
     show InputError, internalError, reportCrash, resetCrashReporting;
 
 import '../util/relativize.dart' show relativizeUri;
 
 import '../compiler_context.dart' show CompilerContext;
 
-import '../util/relativize.dart' show relativizeUri;
-
 import 'kernel_builder.dart'
     show
         Builder,
         ClassBuilder,
         DynamicTypeBuilder,
         InvalidTypeBuilder,
         KernelClassBuilder,
         KernelLibraryBuilder,
         KernelNamedTypeBuilder,
         KernelProcedureBuilder,
@@ skipping 526 matching lines @@
   }
   for (Constructor constructor in superclass.constructors) {
     if (constructor.name.name.isEmpty) {
       return constructor.function.requiredParameterCount == 0
           ? constructor
           : null;
     }
   }
   return null;
 }