Suggestion: Improve typechecker error for refinement on container keys

### HHVM Version

```
HipHop VM 4.13.2 (rel)
Compiler: 1573166016_N
Repo schema: 12f66242996fabd00f4fab1e33c4fbc011981c09
```

### Standalone code, or other way to reproduce the problem

```
function example(dict $foo): void {
if ($foo['bar'] is Traversable<_>) {
foreach ($foo['bar'] as $it) {
// do something
}
}
}
```

### Actual result

```
Typing[4110] Invalid foreach
--> test.hack
1 | function example(dict $foo): void {
| ^^^^^ But got mixed
3 | foreach ($foo['bar'] as $it) {
| ^^^^^^^^^^^
| ^^^^^^^^^^^ Expected Traversable<[unresolved]> because this is used in a foreach statement

1 error found.
```

### Expected result

This is something that consistently trips up new Hack developers as they learn about type refinement and `is/as`. The most ideal state would be that the typechecker could remember the refinement on a sub-key of a container. But I expect that's a significant change. A perhaps much smaller change would be to provide a clearer error message about how to fix this problem, such as recommending the user pull `$foo['bar']` into a local variable before refining its type with the type test.