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
stringToGradient.clear();
		staleGradients.clear();				
	}
	/* (non-Javadoc)
	 * @see org.eclipse.jface.resource.ResourceRegistry#hasValueFor(java.lang.String)
	 */
	public boolean hasValueFor(String gradientKey) {
		return stringToGradientData.containsKey(gradientKey);
	}
	/**
	 * Hook a dispose listener on the SWT display.
	 */
	private void hookDisplayDispose() {
		display.disposeExec(displayRunnable);
	}
	/**
	 * Adds (or replaces) a gradient to this gradient registry under the given 
	 * symbolic name.
	 * <p>
	 * A property change event is reported whenever the mapping from a symbolic
	 * name to a gradient changes. The source of the event is this registry; the
	 * property name is the symbolic gradient name.
	 * </p>
	 * 
	 * @param symbolicName the symbolic gradient name
	 * @param gradientData an <code>GradientData</code> object
	 */
	public void put(String symbolicName, GradientData gradientData) {
		put(symbolicName, gradientData, true);
	}