Set return type of factory constructors properly.

pkg/front_end/lib/src/fasta/kernel/body_builder.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.body_builder;
 
 import '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
 import '../parser/parser.dart' show FormalParameterType, MemberKind, optional;
@@ skipping 464 matching lines @@
     debugEvent("NoInitializers");
   }
 
   @override
   void endInitializers(int count, Token beginToken, Token endToken) {
     debugEvent("Initializers");
   }
 
   DartType _computeReturnTypeContext(MemberBuilder member) {
     if (member is KernelProcedureBuilder) {
-      if (member.target.kind == ProcedureKind.Factory) {
-        return computeFactoryConstructorReturnType(member.target);
-      }

[ahe, 2017/06/01 09:10:08]

I have to "steal" this :-)

       return member.target.function.returnType;
     } else {
       assert(member is KernelConstructorBuilder);
       return null;
     }
   }
 
   @override
   void finishFunction(
       FormalParameters formals, AsyncMarker asyncModifier, Statement body) {
@@ skipping 2737 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }