Fix coverage for define-assertion promise handling

lib/add.test.js

@@ -1,6 +1,7 @@
 "use strict";
 
 var assert = require("assert");
+var sinon = require("sinon");
 var referee = require("./referee");
 
 describe("add", function() {
@@ -130,4 +131,30 @@ describe("add", function() {
             assert(error instanceof TypeError);
         });
     });
+
+    context("with assert returning a promise", function() {
+        before(function() {
+            if (typeof Promise === "undefined") {
+                this.skip();
+            }
+        });
+
+        it("returns a promise and calls referee.pass when resolved", function() {
+            var then = sinon.fake();
+            var options = {
+                assert: function(thing) {
+                    return thing && new Promise(then);
+                },
+                assertMessage: "returning a promise",
+                refuteMessage: "not returning a promise"
+            };
+            referee.add("returningPromise", options);
+
+            var result = referee.assert.returningPromise(1);
+
+            assert(result instanceof Promise);
+
+            then.firstCall.args[0](); // Get coverage on then handler
+        });
+    });
 });