It seems designer does not support provided fields when they need a generator.
I would include the LoginStyle.css stuff in the Login.ui.xml
Something like this:
<ui:UiBinder xmlns:ui='urn:ui:com.google.gwt.uibinder'
xmlns:g='urn:import:com.google.gwt.user.client.ui'>
<ui:style>
.container {
}
</ui:style>
<g:FlowPanel ui:field="container" addStyleNames="{style.container}">
</g:FlowPanel>
</ui:UiBinder>