Replaces META_KEY with `__ember_meta__`

V8 deoptimizes access to the object property:

```javascript
function finishChains() {
  // Access to `obj[META_KEY]` will be deoptimized
  var m = obj[META_KEY], chains = m && m.chains;
  if (chains) {
    // ...
  }
}
```

[Relevant V8 trace](https://gist.github.com/twokul/00b0369dbe86f1ffa011#file-code-asm-L630-L640)
[Hygrogen trace](https://gist.github.com/twokul/dad7e4b671235bd64284#file-hydrogen-cfg-L23837)