Bug 75711 AstProvider should not create AST if requested by SelectionListenerWithASTManager
3.1 M2 If SelectionListenerWithASTManager waits for an AST and the editor closes we should not create an AST for that one but return null instead.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/ASTProvider.java
/**
	 * Tells whether this class is in debug mode.
	 * @since 3.0
	 */
	private static final boolean DEBUG= "true".equalsIgnoreCase(Platform.getDebugOption("org.eclipse.jdt.ui/debug/ASTProvider"));  
	
	/**
	 * Internal activation listener.
	 * 
	 * @since 3.0
	 */
	private class ActivationListener implements IPartListener2, IWindowListener {
		
		/*
		 * @see org.eclipse.ui.IPartListener2#partActivated(org.eclipse.ui.IWorkbenchPartReference)
		 */
		public void partActivated(IWorkbenchPartReference ref) {
			if (isJavaEditor(ref) && !isActiveEditor(ref))
				activeJavaEditorChanged(ref.getPart(true));
		}
		
		/*
		 * @see org.eclipse.ui.IPartListener2#partBroughtToTop(org.eclipse.ui.IWorkbenchPartReference)
		 */
		public void partBroughtToTop(IWorkbenchPartReference ref) {
			if (isJavaEditor(ref) && !isActiveEditor(ref))
				activeJavaEditorChanged(ref.getPart(true));
		}
		
		/*