Enable override ArcanistXHPASTLinter

Summary:
add method setXHPASTTreeForPath() so that a child class of
ArcanistXHPASTLinter can set the tree easily.

Test Plan: wrote a subclass of ArcanistXHPASTLinter and it worked.

Reviewers: pad, epriestley

Reviewed By: pad

CC: aran, pad, jungejason

Differential Revision: 1134

src/lint/linter/xhpast/ArcanistXHPASTLinter.php

@@ -23,7 +23,7 @@
  */
 class ArcanistXHPASTLinter extends ArcanistLinter {
 
-  private $trees = array();
+  protected $trees = array();
 
   const LINT_PHP_SYNTAX_ERROR         = 1;
   const LINT_UNABLE_TO_PARSE          = 2;