JDK9 JavaDoc build fails due to one-character underscore identifiers in hadoop-yarn-common

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.0.0-M1:javadoc (default-cli) on project hadoop-yarn-common: An error has occurred in Javadoc report generation:
[ERROR] Exit code: 1 - ./hadoop/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/webapp/hamlet/Hamlet.java:50: error: as of release 9, '_' is a keyword, and may not be used as an identifier
[ERROR]   public class HTML<T extends _> extends EImp<T> implements HamletSpec.HTML {
[ERROR]                               ^
[ERROR] ./hadoop/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/webapp/hamlet/Hamlet.java:92: error: as of release 9, '_' is a keyword, and may not be used as an identifier
[ERROR]       return base().$href(href)._();
[ERROR]                                 ^
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