Sample.java

public class Sample implements IsWidget {
  private final Binder binder = GWT.create(Binder.class);
  private Widget root;

  @UiField(provided=true) MyBundle bundle = GWT.create(MyBundle.class);

  public Sample() {
    //first, provided items
    bundle.myCss().ensureInjected();

    //then, run the uibinder
    root = binder.createAndBind(this);
  }

  public Widget asWidget() {
    return root;
  }

}

Sample.ui.xml

<ui:UiBinder xmlns:ui='urn:ui:com.google.gwt.uibinder' ...>

  <!-- because we call it bundle here... -->
  <ui:with type="package.to.my.client.MyBundle" field="bundle" />

  <!-- ... we can reference it here -->
  <g:Widget addStyleName="{bundle.myCss.style}"... />
</ui:UiBinder>