code: public @Nonnull
    StringAssert contains(@Nonnull String sequence) {
      strings.assertContains(description, actual, sequence);
      return this;
    }
docs: Verifies that the <em>actual</em> {@code String} contains the given sequence.
  
  @param sequence the sequence to look for.
  @return {@code this} assertion object.
  @throws NullPointerException if the given sequence is {@code null}.
  @throws AssertionError if the <em>actual</em> {@code String} is {@code null}.
  @throws AssertionError if the <em>actual</em> {@code String} does not contain the given one.