closure-compiler skips type checks if output language level is higher than the type checker understands

Create the following `main.js`:

```
/** @return {string} */
var main = function() { return 2; };
```

And run the following commandline:

```
java -jar closure-compiler-v20180610.jar --js main.js --jscomp_error checkTypes --language_in ECMASCRIPT_2017 --language_out ECMASCRIPT_2017
```

Expectation:
closure-compiler should fail with a type error "inconsistent return type"

Actual:
closure-compiler prints `WARNING: Skipping pass checkTypes` (!!!) but otherwise passes successfully

Note that this works as expected if you pass `--language_out ECMASCRIPT5`