Compute correct return type for factory methods.

pkg/front_end/lib/src/fasta/source/source_library_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.source_library_builder;
 
 import 'package:front_end/src/scanner/token.dart' show Token;
 
 import 'package:kernel/ast.dart' show ProcedureKind;
 
@@ skipping 12 matching lines @@
 import '../builder/builder.dart'
     show
         Builder,
         ClassBuilder,
         ConstructorReferenceBuilder,
         FormalParameterBuilder,
         FunctionTypeBuilder,
         LibraryBuilder,
         MemberBuilder,
         MetadataBuilder,
+        NamedTypeBuilder,
         PrefixBuilder,
         ProcedureBuilder,
         Scope,
         TypeBuilder,
         TypeDeclarationBuilder,
         TypeVariableBuilder,
         Unhandled;
 
 abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
     extends LibraryBuilder<T, R> {
@@ skipping 490 matching lines @@
       factoryDeclarations.forEach((_, DeclarationBuilder<T> declaration) {
         parent.types.addAll(declaration.types);
       });
       parent.types.addAll(types);
     } else {
       factoryDeclarations.forEach(
           (ProcedureBuilder procedure, DeclarationBuilder<T> declaration) {
         assert(procedure.typeVariables.isEmpty);
         procedure.typeVariables
             .addAll(library.copyTypeVariables(typeVariables));
+        DeclarationBuilder<T> savedDeclaration = library.currentDeclaration;
+        library.currentDeclaration = declaration;
+        for (TypeVariableBuilder tv in procedure.typeVariables) {
+          NamedTypeBuilder<T, dynamic> t = procedure.returnType;
+          t.arguments
+              .add(library.addNamedType(tv.name, null, procedure.charOffset));
+        }
+        library.currentDeclaration = savedDeclaration;
         declaration.resolveTypes(procedure.typeVariables, library);
       });
       Map<String, TypeVariableBuilder> map = <String, TypeVariableBuilder>{};
       for (TypeVariableBuilder builder in typeVariables) {
         map[builder.name] = builder;
       }
       for (T type in types) {
         String name = type.name;
         TypeVariableBuilder builder;
         if (name != null) {
@@ skipping 16 matching lines @@
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 
   Scope toScope(Scope parent) {
     return new Scope(members, setters, parent, isModifiable: false);
   }
 }