Set 'isSyntheticDefault' for default constructors.

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 'package:kernel/ast.dart'
     show
@@ skipping 488 matching lines @@
         returnType: const VoidType());
     SuperInitializer initializer = new SuperInitializer(
         constructor, new Arguments(positional, named: named));
     return new Constructor(function,
         name: constructor.name, initializers: <Initializer>[initializer]);
   }
 
   Constructor makeDefaultConstructor() {
     return new Constructor(
         new FunctionNode(new EmptyStatement(), returnType: const VoidType()),
-        name: new Name(""));
+        name: new Name(""),
+        isSyntheticDefault: true);
   }
 
   void finishAllConstructors() {
     Class objectClass = this.objectClass;
     for (SourceClassBuilder builder in collectAllSourceClasses()) {
       Class cls = builder.target;
       if (cls != objectClass) {
         finishConstructors(builder);
       }
     }
@@ skipping 135 matching lines @@
   }
   for (Constructor constructor in superclass.constructors) {
     if (constructor.name.name.isEmpty) {
       return constructor.function.requiredParameterCount == 0
           ? constructor
           : null;
     }
   }
   return null;
 }