Wrong error message for `this` in function

**TypeScript Version:**  3.1.0-dev.20180810

**Code**

```ts
function f() {
    this.x;
}
```

**Expected behavior:**

Error that `this` is implicitly `any`.

**Actual behavior:**

```
src/a.ts:2:5 - error TS7041: The containing arrow function captures the global value of 'this' which implicitly has type 'any'.
```

...what arrow function?
(Ref: #25978)