Mark native methods as external.

test/kernel/regression/native_is_illegal.dart.txt

Index: test/kernel/regression/native_is_illegal.dart.txt
diff --git a/test/kernel/regression/native_is_illegal.dart.txt b/test/kernel/regression/native_is_illegal.dart.txt
new file mode 100644
index 0000000000000000000000000000000000000000..36cec9b05d3bf5f9ce7639dff4ff1f2ea29ed433
--- /dev/null
+++ b/test/kernel/regression/native_is_illegal.dart.txt
@@ -0,0 +1,16 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Bar extends core::Object {
+  get x() → self::Bar
+    return "Bar_get_x";
+  set x(self::Bar value) → dynamic
+    return "Bar_set_x";
+  method f() → dynamic
+    return "Bar_f";
+  static factory •() → self::Bar
+    return "Bar_constructor";
+}
+static method foo() → dynamic
+  invalid-statement;