dubbo registry extension compatibility with dubbo 2.6.x

[cvictory]

I write a registry extension in dubbo 2.6. Now I import it into a project that depend on dubbo2.7, some error happened.

Caused by: java.lang.NoClassDefFoundError: com/alibaba/dubbo/registry/support/AbstractRegistryFactory
	at java.lang.ClassLoader.defineClass1(Native Method) ~[na:1.8.0_181]
	at java.lang.ClassLoader.defineClass(ClassLoader.java:763) ~[na:1.8.0_181]
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[na:1.8.0_181]
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:467) ~[na:1.8.0_181]
	at java.net.URLClassLoader.access$100(URLClassLoader.java:73) ~[na:1.8.0_181]
	at java.net.URLClassLoader$1.run(URLClassLoader.java:368) ~[na:1.8.0_181]
	at java.net.URLClassLoader$1.run(URLClassLoader.java:362) ~[na:1.8.0_181]
	at java.security.AccessController.doPrivileged(Native Method) ~[na:1.8.0_181]
	at java.net.URLClassLoader.findClass(URLClassLoader.java:361) ~[na:1.8.0_181]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_181]
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349) ~[na:1.8.0_181]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_181]
	at java.lang.Class.forName0(Native Method) ~[na:1.8.0_181]
	at java.lang.Class.forName(Class.java:348) ~[na:1.8.0_181]
	at org.apache.dubbo.common.extension.ExtensionLoader.loadResource(ExtensionLoader.java:702) ~[dubbo-2.7.1.jar:2.7.1]
	... 32 common frames omitted
Caused by: java.lang.ClassNotFoundException: com.alibaba.dubbo.registry.support.AbstractRegistryFactory
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[na:1.8.0_181]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_181]
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349) ~[na:1.8.0_181]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_181]
	... 47 common frames omitted