[ES6] Pattern params default values

Per latest ES6 draft [1], function parameters that are destructuring patterns
may have default values. This diff allows parsing `AssignmentExpression`
that follows the pattern and `=`.

See tests attached for details.

[1] https://people.mozilla.org/~jorendorff/es6-draft.html#sec-function-definitions

```
FormalParameter : BindingElement

BindingElement : BindingPattern Initializer[opt]
```