JDK 13 RFR of JDK-8224174: java.lang.Number has a default constructor

[Joe Darcy]

Hello,

While doing some other work, I noticed that java.lang.Number relies on 
an implicit default constructor. Until there is a lint warning to find 
these cases systematically (JDK-8071961: Add javac lint warning when a 
default constructor is created), please review the diff below to address 
this particular instance.

Thanks,

-Joe


  public abstract class Number implements java.io.Serializable {
      /**
+     * Constructor for subclasses to call.
+     */
+    public Number() {super();}
+
+    /**