Correctly infer the return context while compiling a factory constructor.

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 450 matching lines @@
   @override
   void handleNoInitializers() {
     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);
+      }
+      return member.target.function.returnType;
+    } else {
+      assert(member is KernelConstructorBuilder);
+      return null;
+    }
+  }
+
   @override
   void finishFunction(
       FormalParameters formals, AsyncMarker asyncModifier, Statement body) {
     debugEvent("finishFunction");
     typePromoter.finished();
     _typeInferrer.inferFunctionBody(
-        member.target.function.returnType, asyncModifier, body);
+        _computeReturnTypeContext(member), asyncModifier, body);
     KernelFunctionBuilder builder = member;
     builder.body = body;
     if (formals?.optional != null) {
       Iterator<FormalParameterBuilder> formalBuilders =
           builder.formals.skip(formals.required.length).iterator;
       for (VariableDeclaration parameter in formals.optional.formals) {
         bool hasMore = formalBuilders.moveNext();
         assert(hasMore);
         VariableDeclaration realParameter = formalBuilders.current.target;
         Expression initializer =
@@ skipping 2691 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }