Warn on [] lookup on enums

Currently, you can make this mistake of doing something like:

``` js
/**
* @enum {number}
*/
some.ns.THING = {
FOO: 1
}
```

and try to do:

``` js
var someString = // get some string from somewhere and the value is FOO
var value = some.ns.THING[someString];

// think value is 1 and try to use it as such
```

As a result, value ends up being undefined when the compiler renames things. If a problem with the undefined value happens down the line, it can be hard to track down that the enum lookup shouldn't have been done.

It would be really nice if `[]` lookup could warn (or error) on things that are known to be enums. I can't really think of a good case where you would capture the enum keys (e.g. via `Object.keys`) and try to use them later to get the actual enum value.