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
* @return <code>true</code if the given AST is the cached one
	 */
	public boolean isCached(CompilationUnit ast) {
		return ast != null && fAST == ast; 
	}
	/**
	 * Informs that reconciling for the given element is about to be started.
	 * 
	 * @param javaElement the Java element
	 * @see org.eclipse.jdt.internal.ui.text.java.IJavaReconcilingListener#aboutToBeReconciled()
	 */
	void aboutToBeReconciled(IJavaElement javaElement) {
		if (DEBUG)
			System.out.println(DEBUG_PREFIX + "about to reconcile: " + toString(javaElement)); 
		if (javaElement == null)
			return;
		
		synchronized (fReconcileLock) {
			fIsReconciling= true;
			fReconcilingJavaElement= javaElement;
		}
		cache(null, javaElement);
	}
	
	/**
	 * Disposes the cached AST.
	 */
	private synchronized void disposeAST() {
		
		if (fAST == null)