Bug 337002 EPartService leaks previously active part
A reference is kept to the previously active part. This is unnecessary. We only need it to fire events. After the deactivation events have been fired, we should no longer need to keep it lying around.
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/application/EPartServiceTest.java
.createPerspective();
		perspectiveStack.getChildren().add(perspectiveB);
		MPlaceholder areaPlaceholderB = AdvancedFactoryImpl.eINSTANCE
				.createPlaceholder();
		areaPlaceholderB.setRef(area);
		perspectiveB.getChildren().add(areaPlaceholderB);
		perspectiveB.setSelectedElement(areaPlaceholderB);
		initialize(applicationContext, application);
		getEngine().createGui(window);
		EPartService partService = window.getContext().get(EPartService.class);
		partService.activate(sharedPart);
		perspectiveStack.setSelectedElement(perspectiveB);
		assertEquals(sharedPart, partService.getActivePart());
		partService.hidePart(sharedPart);
		assertNull(partService.getActivePart());
		perspectiveStack.setSelectedElement(perspectiveA);
		assertEquals(partA, partService.getActivePart());
	}
	/**
	 * Test to ensure that switching perspectives doesn't cause a hidden shared
	 * part to be displayed again.
	 */
	public void testHidePart_ActivationHistory16A() {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MWindow window = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(window);
		application.setSelectedElement(window);
		MPart partA = BasicFactoryImpl.eINSTANCE.createPart();
		window.getSharedElements().add(partA);