8054717: SJavac should track changes in the public apis of classpath classes!

Added functionality for tracking changes in public APIs of classpath classes.

Reviewed-by: jlahoda, erikj

langtools/test/tools/sjavac/IncCompInheritance.java

@@ -60,6 +60,7 @@ public class IncCompInheritance extends SjavacBase {
             throw new AssertionError("Compilation failed unexpectedly");
 
         // Remove method A.m
+        Thread.sleep(2500); // Make sure we get a new timestamp
         String aModified = "package pkga; public class A { }";
         toolbox.writeFile(src.resolve("pkga/A.java"), aModified);