vaadin-framework/checkstyle/import-control.xml

<?xml version="1.0"?>
<!DOCTYPE import-control PUBLIC
    "-//Puppy Crawl//DTD Import Control 1.1//EN"
    "http://www.puppycrawl.com/dtds/import_control_1_1.dtd">

<import-control pkg="com.vaadin">
    <allow pkg=".*" regex="true" />
    <disallow class="com.google.gwt.user.client.Element" />
</import-control>