Let parser handle factory modifiers.

pkg/front_end/lib/src/fasta/kernel/kernel_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.kernel_library_builder;
 
 import 'package:kernel/ast.dart';
 
 import 'package:kernel/clone.dart' show
     CloneVisitor;
@@ skipping 149 matching lines @@
       procedure = new KernelProcedureBuilder(metadata, modifiers, returnType,
           name, typeVariables, formals, asyncModifier, kind, this, charOffset,
           nativeMethodName);
     }
     addBuilder(name, procedure, charOffset);
     if (nativeMethodName != null) {
       addNativeMethod(procedure);
     }
   }
 
-  void addFactoryMethod(List<MetadataBuilder> metadata,
+  void addFactoryMethod(List<MetadataBuilder> metadata, int modifiers,
       ConstructorReferenceBuilder constructorName,
       List<FormalParameterBuilder> formals, AsyncMarker asyncModifier,
       ConstructorReferenceBuilder redirectionTarget, int charOffset,
       String nativeMethodName) {
     // Nested declaration began in `OutlineBuilder.beginFactoryMethod`.
     DeclarationBuilder<KernelTypeBuilder> factoryDeclaration =
         endNestedDeclaration();
     String name = constructorName.name;
     int index = name.indexOf(".");
     name = index == -1 ? "" : name.substring(index + 1);
     assert(constructorName.suffix == null);
     KernelProcedureBuilder procedure = new KernelProcedureBuilder(metadata,
-        staticMask, null, name, <TypeVariableBuilder>[], formals, asyncModifier,
-        ProcedureKind.Factory, this, charOffset, nativeMethodName,
-        redirectionTarget);
+        staticMask | modifiers, null, name, <TypeVariableBuilder>[], formals,
+        asyncModifier, ProcedureKind.Factory, this, charOffset,
+        nativeMethodName, redirectionTarget);
     currentDeclaration.addFactoryDeclaration(procedure, factoryDeclaration);
     addBuilder(name, procedure, charOffset);
     if (nativeMethodName != null) {
       addNativeMethod(procedure);
     }
   }
 
   void addEnum(List<MetadataBuilder> metadata, String name,
       List<String> constants, int charOffset) {
     addBuilder(name,
@@ skipping 160 matching lines @@
   }
 }
 
 bool isConstructorName(String name, String className) {
   if (name.startsWith(className)) {
     if (name.length == className.length) return true;
     if (name.startsWith(".", className.length)) return true;
   }
   return false;
 }