>a</code> and <code>b</code> are null, then this method will return null. Otherwise,
any null values for <code>a</code> or <code>b</code> are interpreted as equivalent to <a href="dart-ui/FontWeight/normal-constant.html">normal</a>
(also known as <a href="dart-ui/FontWeight/w400-constant.html">w400</a>).</p>
<p>The <code>t</code> argument represents position on the timeline, with 0.0 meaning
that the interpolation has not started, returning <code>a</code> (or something
equivalent to <code>a</code>), 1.0 mea