ClassCastException: sun.awt.HeadlessToolkit cannot be cast to sun.awt.SunToolkit in headless mode

This is basically an 8u20 mirror of JDK-8055822

For example, the following code throws ClassCastException if run v.s. 8u20b26 (GA) with -Djava.awt.headless=true or even if DISPLAY variable is not set

Tested on Linux

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import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.FlavorMap;
import java.awt.datatransfer.SystemFlavorMap;
import java.util.List;

public class SystemFlavorMapTest {

    public static void main(String[] args) {
        FlavorMap fm = SystemFlavorMap.getDefaultFlavorMap();
        SystemFlavorMap sfm = (SystemFlavorMap)fm;
        final List<DataFlavor> flavorsForNative = sfm.getFlavorsForNative("not-for-sure/registered-before");
        System.err.println("flavorsForNative = " + flavorsForNative);
    }
}

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The following tests which exist in _JCK8_

api/java_awt/datatransfer/DataFlavor/index.html#rclass
api/java_awt/datatransfer/SystemFlavorMap/index.html#GetF4N
api/java_awt/datatransfer/SystemFlavorMap/index.html#SetF4N
api/java_awt/datatransfer/SystemFlavorMap/index.html#SetN4F

fail.

See JCK-7303484 for the test logs

This is likely a regression introduced by JDK-8037202