Bug 53881 [Colors] Color/Gradient registry should return default values.
There should be default values for when a user does .get() on a bad key.
bundles/org.eclipse.jface/src/org/eclipse/jface/resource/GradientRegistry.java
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import org.eclipse.jface.util.Assert;
import org.eclipse.swt.widgets.Display;
/**
 * A gradient registry maintains a mapping between symbolic gradient names and SWT 
 * <code>Gradient</code>s.
 * <p>
 * A gradient registry owns all of the <code>Gradient</code> objects registered with 
 * it, and automatically disposes of them when the SWT Display that creates the 
 * <code>Gradient</code>s is disposed. Because of this, clients do not need to 
 * (indeed, must not attempt to) dispose of <code>Gradient</code> objects 
 * themselves.
 * </p>
 * <p>
 * Methods are provided for registering listeners that will be kept
 * apprised of changes to list of registed gradients.
 * </p>
 * <p>
 * Clients may instantiate this class (it was not designed to be subclassed).
 * </p>
 * 
 * @since 3.0
 */
public class GradientRegistry extends ResourceRegistry{