YARN-8788. mvn package -Pyarn-ui fails on JDK9

This closes #421

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-ui/pom.xml

@@ -202,6 +202,14 @@
             <groupId>ro.isdc.wro4j</groupId>
             <artifactId>wro4j-maven-plugin</artifactId>
             <version>1.7.9</version>
+            <dependencies>
+              <!-- TODO: Remove this dependency after upgrading wro4j-maven-plugin to 1.8.1 or later. -->
+              <dependency>
+                <groupId>org.mockito</groupId>
+                <artifactId>mockito-core</artifactId>
+                <version>2.18.0</version>
+              </dependency>
+            </dependencies>
             <executions>
               <execution>
                 <phase>prepare-package</phase>