Does Dart have a widget library?

dart dart-polymer angular-dart

There are widget libraries but they are mostly work in progress.

for Angular2 Dart

for Angular.dart 1.x

angular.dart.ui a Dart port of Angular UI Polymer
AngularPrime-Dart
Angular.dart.material

for Polymer.dart >= 1.0.0

polymer_elements Dartport of JS https://elements.polymer-project.org/
bwu_datagrid Dart port of JS SlickGrid

for Polymer.dart <= 0.16.x

WebGL

RöckDöt for Dart

others

DEPRECATED

polymer_ui_elements Dart port of JS polymer-ui-elements
core_elements Dart port of JS core-elements (just started)

and probably others

dart dart-polymer angular-dart

Another one worth mentioning:http://www.material-design-lite.pub