Let parser handle factory modifiers.

pkg/front_end/lib/src/fasta/source/outline_builder.dart

Index: pkg/front_end/lib/src/fasta/source/outline_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/outline_builder.dart b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
index 6ad8241fc316f4256944aaa3fda1112a19f2bf65..f4fcc093012cf9165683ef0e36e72d35023c0ac9 100644
--- a/pkg/front_end/lib/src/fasta/source/outline_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
@@ -544,8 +544,9 @@ class OutlineBuilder extends UnhandledListener {
     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;
   }