Use with-current-buffer instead of save-excursion + set-buffer

to fix the deprecation warnings

Closes mooz#28

js2-mode.el

@@ -4512,8 +4512,7 @@ Function also calls `js2-node-add-children' to add the parent link."
         pos)
     (unless buf
       (error "No buffer available for node %s" node))
-    (save-excursion
-      (set-buffer buf)
+    (with-current-buffer buf
       (buffer-substring-no-properties (setq pos (js2-node-abs-pos node))
                                       (+ pos (js2-node-len node))))))
 
@@ -7330,8 +7329,7 @@ leaving a statement, an expression, or a function definition."
         ast)
     (or buf (setq buf (current-buffer)))
     (message nil)  ; clear any error message from previous parse
-    (save-excursion
-      (set-buffer buf)
+    (with-current-buffer buf
       (setq js2-scanned-comments nil
             js2-parsed-errors nil
             js2-parsed-warnings nil