Let parser handle factory modifiers.

pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
index f4d7f4048df8de34d42ac512c828707c07b6f41b..25d01846eb03a3426d806248b603816058621008 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
@@ -167,7 +167,7 @@ class KernelLibraryBuilder
     }
   }
 
-  void addFactoryMethod(List<MetadataBuilder> metadata,
+  void addFactoryMethod(List<MetadataBuilder> metadata, int modifiers,
       ConstructorReferenceBuilder constructorName,
       List<FormalParameterBuilder> formals, AsyncMarker asyncModifier,
       ConstructorReferenceBuilder redirectionTarget, int charOffset,
@@ -180,9 +180,9 @@ class KernelLibraryBuilder
     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) {