Let parser handle factory modifiers.

pkg/front_end/lib/src/fasta/source/outline_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.outline_builder;
 
 import 'package:kernel/ast.dart' show
     AsyncMarker,
     ProcedureKind;
 
@@ skipping 526 matching lines @@
   void endFactoryMethod(Token beginToken, Token endToken) {
     debugEvent("FactoryMethod");
     MethodBody kind = pop();
     ConstructorReferenceBuilder redirectionTarget;
     if (kind == MethodBody.RedirectingFactoryBody) {
       redirectionTarget = pop();
     }
     AsyncMarker asyncModifier = pop();
     List<FormalParameterBuilder> formals = pop();
     var name = pop();
+    int modifiers = Modifier.validate(pop());
     List<MetadataBuilder> metadata = pop();
-    library.addFactoryMethod(metadata, name, formals, asyncModifier,
+    library.addFactoryMethod(metadata, modifiers, name, formals, asyncModifier,
         redirectionTarget, beginToken.charOffset, nativeMethodName);
     nativeMethodName = null;
   }
 
   @override
   void endRedirectingFactoryBody(Token beginToken, Token endToken) {
     debugEvent("RedirectingFactoryBody");
     push(MethodBody.RedirectingFactoryBody);
   }
 
@@ skipping 65 matching lines @@
   Link<Token> handleMemberName(Link<Token> identifiers) {
     if (!isDartLibrary || identifiers.isEmpty) return identifiers;
     return removeNativeClause(identifiers);
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }