1. Save a.el and b.el to the same dir.

2. Open b.el, type M-x eval-buffer.

3. Evaluate (b-foo), see 4 in the echo area.

Note that the assertion in `b-bar' succeeds, and so while `a-a' is not 
globally bound, it's visible from `b-bar'.

If I replace the definition of b-bar with

```
(defvar a-a)

(defun b-bar ()
   (cl-assert (= a-a 4))
   (let ((a-a 5))
     (a-bar)))
```

or even with

```
(defun b-bar ()
   (defvar a-a)
   (cl-assert (= a-a 4))
   (let ((a-a 5))
     (a-bar)))
```

(which apparently doesn't make `a-a' globally defined in that file, ref. 
http://debbugs.gnu.org/18059)

then, as I'd expect, after M-x eval-buffer, (b-bar) evaluates to 5.

Why does this happen? Is (defvar foo) the recommended option to use with 
lexical-binding on?

-- Dmitry