Vaadin Javadoc issues

General
1 
C:\Users\xxxx\.gradle\caches\modules-2\files-2.1\com.vaadin.external.gwt\gwt-elemental\2.8.2.vaadin2\abdfd4fcfbc0652298061be8c70a56a6d6e868b2\gwt-elemental-2.8.2.vaadin2.jar(/elemental/json/impl/JreJsonArray.java):180: error: package com.google.gwt
.core.shared does not exist
  @com.google.gwt.core.shared.GwtIncompatible

Is one of 14 error I’m gettin when trying to create javadoc.
I’m using gradle with javadoc as follows:

javadoc {
	options.encoding = 'UTF-8'
	options.docEncoding = 'UTF-8'
	options.charSet = 'UTF-8'
	source = sourceSets.main.allJava
	classpath = configurations.runtimeClasspath
}

I don’t have any idea how to fix it… Anyone ?