getBindings doesn't detect a closure's free variables

```javascript
> var recast = require("recast");
undefined
> recast.visit(recast.parse("var a, bb; bb = 3; var x = function() { bb = 4; };"), { visitIdentifier: function(path) {
console.log(path);
console.log(path.scope.getBindings());
console.log(path.scope.getBindings().hasOwnProperty(path.node.name));
console.log("====");
this.traverse(path);
}});
[..]
[..]
[..]
====
NodePath {
value:
Identifier {
type: 'Identifier',
name: 'bb',
loc: { start: [Object], end: [Object], lines: Lines {}, indent: 0 } },
parentPath:
NodePath {
value:
AssignmentExpression {
type: 'AssignmentExpression',
operator: '=',
[..]
name: 'left',
__childCache: null }
{}
false
====
{ program:
[..]
type: 'File',
comments: null }
```

Start with the getBindings implementation and reproduce the issue using the JavaScript snippet in the report. Trace the scope handling for the function assigned to x, then verify that the closure's reference to bb is detected as a free variable and that the existing binding checks reflect it.

Written by the indexing model from the issue text.