Correctly infer the return context while compiling a factory constructor.

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index a5efbc240083ed52a1021824b5fba8b950824e25..919d44433af6e99a08e02d7953e7e3bfbda49917 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -468,13 +468,25 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
     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) {