Add test for compatibility

Author: gibachan

test/Compatibility/dot_keywords.swift

@@ -0,0 +1,13 @@
+// RUN: %target-typecheck-verify-swift -swift-version 3
+
+// Static function in protocol should have `Self.` instead of its protocol name
+protocol P {}
+
+extension P {
+  static func f1() {}
+
+  func g() {
+    f1() // expected-error {{use of unresolved identifier 'f1'}}
+  }
+}
+