title: DateTime.fromMillisecondsSinceEpoch seems wrong
body: <p>From the <a href="https://api.dart.dev/stable/2.3.0/dart-core/DateTime/DateTime.fromMillisecondsSinceEpoch.html" rel="nofollow noreferrer">documentation</a></p>
  <blockquote>
  <p>The constructed DateTime represents 1970-01-01T00:00:00Z + millisecondsSinceEpoch ms in the given time zone (local or UTC).</p>
  </blockquote>
  <p>Therefore, if my local timezone is +1, this test should pass -</p>
  <pre class="lang-dart prettyprint-override"><code>test('DateTime', () {
    var dt = DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
    expect(dt.toIso8601String(), '1970-01-01T00:00:00.000Z');
  
    var dtLocal = DateTime.fromMillisecondsSinceEpoch(0, isUtc: false);
    expect(dtLocal.toIso8601String(), '1970-01-01T00:00:00.000');
  });
  </code></pre>
  <p>However, it fails as <code>dtLocal.toIso8601String()</code> gives <code>1970-01-01T01:00:00.000</code>. Is it just me or is the documentation unclear? I would expect it to just change the timezone, not the milliseconds since epoch based on the local timezone.</p>