Compiler options have different effects on command line vs in fourslash test

**TypeScript Version:**  Version 3.0.0-dev.20180616

**Code**

```ts
/// <reference path="fourslash.ts" />

// @target: esnext

////Symbol.iterator;

verify.noErrors();
```

This fourslash test fails. But putting `Symbol.iterator;` in a file and running `tsc a.ts --target esnext` compiles with no error. `// @lib: esnext` doesn't work either, even though `tsc a.ts --lib esnext` works.