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
```

if (skipFailback) {
                    t = t.getCause();
                }
                throw new IllegalStateException("Failed to register " + url + " to registry " + getUrl().getAddress() + ", cause: " + t.getMessage(), t);
            } else {
                logger.error("Failed to register " + url + ", waiting for retry, cause: " + t.getMessage(), t);
            }
            
            addFailedRegistered(url);
        }
    }
    @Override
    public void unregister(URL url) {
        super.unregister(url);
        removeFailedRegistered(url);
        removeFailedUnregistered(url);
        try {
            
            doUnregister(url);
        } catch (Exception e) {
            Throwable t = e;
            
            boolean check = getUrl().getParameter(Constants.CHECK_KEY, true)
                    && url.getParameter(Constants.CHECK_KEY, true)
                    && !CONSUMER_PROTOCOL.equals(url.getProtocol());
            boolean skipFailback = t instanceof SkipFailbackWrapperException;
            if (check || skipFailback) {
                if (skipFailback) {
                    t = t.getCause();
                }