Throw Expressions (Stage 2)

Champions: Ron Buckton (@rbuckton)
Spec Repo: https://github.com/rbuckton/proposal-throw-expressions
First presented at the July 2017 meeting: https://github.com/tc39/agendas/blob/master/2017/07.md
Stage 1 to Stage 2 at Sept 2017 meeting

Examples:

```js
function save(filename = throw new TypeError("Argument required")) {}
```
```js
lint(ast, {
with: () => throw new Error("avoid using 'with' statements.")
});
```
```js
function getEncoder(encoding) {
const encoder = encoding === "utf8" ? new UTF8Encoder()
: encoding === "utf16le" ? new UTF16Encoder(false)
: encoding === "utf16be" ? new UTF16Encoder(true)
: throw new Error("Unsupported encoding");
}
```
```js
class Product {
get id() { return this._id; }
set id(value) { this._id = value || throw new Error("Invalid value"); }
}
```

## Implementation

- [x] Parser
- [x] Transform
- TS: https://github.com/Microsoft/TypeScript/pull/18798
- REPL: https://babeljs.io/repl/build/master#?&browsers=&build=&code_lz=DYSwdgLgFAhgzhANAAgN7IFDOQdxBACwC5koBKZAXgD5lCAnAex2TAFMWBRep-qAIhgA3RiAAmyAK5xwAc2QByPIQXIEMCGwC2bSHAB0_MhgC-ZANwYgA&circleciRepo=&lineWrap=true&presets=stage-0&targets=&version=7.0.0-beta.2

Start with the linked proposal repository and the issue's Implementation checklist. Review the parser and transform entries, then compare the TypeScript pull request and Babel REPL links with the current proposal status. The issue does not define a remaining change or a clear completion condition.

Written by the indexing model from the issue text.