Bug 1326534 - Add assert.in for own properties; r=automatedtester

The implementation of the new library function `assert.in` is analogous
to Python's `unittest.TestCase.assertIn`.

MozReview-Commit-ID: 1pjS9ttPXgo

--HG--
extra : rebase_source : 3f4121226d65f127fdf0c990915483659f3a7f93

testing/marionette/assert.js

@@ -212,6 +212,28 @@ assert.object = function (obj, msg = "") {
   return assert.that(o =>
       Object.prototype.toString.call(o) == "[object Object]", msg)(obj);
 };
+
+/**
+ * Asserts that |prop| is in |obj|.
+ *
+ * @param {?} prop
+ *     Own property to test if is in |obj|.
+ * @param {?} obj
+ *     Object.
+ * @param {string=} msg
+ *     Custom error message.
+ *
+ * @return {?}
+ *     Value of |obj|'s own property |prop|.
+ *
+ * @throws {InvalidArgumentError}
+ *     If |prop| is not in |obj|, or |obj| is not an object.
+ */
+assert.in = function (prop, obj, msg = "") {
+  assert.object(obj, msg);
+  msg = msg || error.pprint`Expected ${prop} in ${obj}`;
+  assert.that(p => obj.hasOwnProperty(p), msg)(prop);
+  return obj[prop];
 };
 
 /**

testing/marionette/test_assert.js

@@ -76,6 +76,15 @@ add_test(function test_object() {
   run_next_test();
 });
 
+add_test(function test_in() {
+  assert.in("foo", {foo: 42});
+  for (let typ of [{}, 42, true, null, undefined]) {
+    Assert.throws(() => assert.in("foo", typ), InvalidArgumentError);
+  }
+
+  run_next_test();
+});
+
 add_test(function test_array() {
   assert.array([]);
   assert.array(new Array());