If it intentionally accepts both forms then that should be clarifed in javadoc.

Please see the minimized test below to reproduce the issue.

Minimized test:
===============
$ cat Test.java
import java.lang.invoke.*;

public class Test {
    public static void main(String[] args) throws Exception {
        for (String d : new String[] {"(Ljava/lang/Object;)V", "(Ljava.lang.Object;)V"}) {
            MethodType.fromMethodDescriptorString(d, null);
            System.out.println("OK");
        }
    }
}

Minimized test output:
========================
$ javac Test.java

$ java -showversion -verify Test
java version "1.7.0-ea"
Java(TM) SE Runtime Environment (build 1.7.0-ea-b138)
Java HotSpot(TM) Server VM (build 21.0-b08, mixed mode)

OK
OK