code: public <K> void assertContainsKey(Description description, Map<?, ?> actual, K key) {
      assertNotNull(description, actual);
      if (!actual.containsKey(key)) {
        throw failures.failure(description, shouldContainKey(actual, key));
      }
    }
docs: Asserts that the actual map contain the given key.
  
  @param description contains information about the assertion.
  @param actual the given {@code Map}.
  @param key the given key
  @throws AssertionError if the actual map is {@code null}.
  @throws AssertionError if the actual map not contains the given key.