Current behavior

Flutter allows obscured text fields to be multi-line, but it prevents new lines from being added to the text.

Motivation

Passwords are typically entered in single-line fields. It doesn't make sense to have a password field with multiple lines.

Another reason is that the web supports obscuring text only on single-line text fields (aka <input type="password">).

Proposed change

Throw if EditableText.obscureText is true and EditableText.maxLines is greater than 1.