Trigger a checkstyle error if importing the wrong Element class (#13287)

Change-Id: Ief15e88c9c6fe5fd88dbf9f12783ac2f5ae617c6

checkstyle/import-control.xml

@@ -0,0 +1,9 @@
+<?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>

checkstyle/vaadin-checkstyle.xml

@@ -142,6 +142,10 @@
         <module name="IllegalImport" /> <!-- defaults to sun.* packages -->
         <module name="RedundantImport" />
         <module name="UnusedImports" />
+        <module name="ImportControl">
+            <property name="file" value="${checkstyle.importControl.file}" />
+            <property name="severity" value="error" />
+        </module>
 
 
         <!-- Checks for Size Violations. -->

common.xml

@@ -422,6 +422,7 @@
     <property name="cs.dir" location="${vaadin.basedir}/checkstyle" />
     <property name="cs.xml" location="${cs.dir}/vaadin-checkstyle.xml" />
     <property name="cs.header-file" location="${cs.dir}/header" />
+    <property name="cs.import-control-file" location="${cs.dir}/import-control.xml" />
     <taskdef resource="checkstyletask.properties" uri="antlib:com.puppycrawl.tools.checkstyle" classpathref="taskdefs.classpath" />
 
     <target name="checkstyle">
@@ -438,6 +439,7 @@
             </fileset>
             <formatter type="xml" toFile="${result.dir}/checkstyle-errors.xml" />
             <property key="checkstyle.header.file" file="${cs.header-file}" />
+            <property key="checkstyle.importControl.file" file="${cs.import-control-file}" />
         </cs:checkstyle>
     </target>
 </project>