Let parser handle factory modifiers.

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:kernel/ast.dart' show
     AsyncMarker,
     ProcedureKind;
 
@@ skipping 174 matching lines @@
       {bool isTopLevel});
 
   void addEnum(List<MetadataBuilder> metadata, String name,
       List<String> constants, int charOffset);
 
   void addFunctionTypeAlias(List<MetadataBuilder> metadata,
       T returnType, String name,
       List<TypeVariableBuilder> typeVariables,
       List<FormalParameterBuilder> formals, int charOffset);
 
-  void addFactoryMethod(List<MetadataBuilder> metadata,
+  void addFactoryMethod(List<MetadataBuilder> metadata, int modifiers,
       ConstructorReferenceBuilder name, List<FormalParameterBuilder> formals,
       AsyncMarker asyncModifier, ConstructorReferenceBuilder redirectionTarget,
       int charOffset, String nativeMethodName);
 
   FormalParameterBuilder addFormalParameter(
       List<MetadataBuilder> metadata, int modifiers,
       T type, String name, bool hasThis, int charOffset);
 
   TypeVariableBuilder addTypeVariable(String name, T bound, int charOffset);
 
@@ skipping 266 matching lines @@
   }
 
   /// Called to register [procedure] as a factory whose types are collected in
   /// [factoryDeclaration]. Later, once the class has been built, we can
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 }