Document how babel-plugin-transform-runtime changes references to builtins as a pitfall

I am trying to use reflection and Typescript decorators. My library uses the reflect-metadata polyfill. The babel-plugin-transform-runtime plugin is clobbering the references to Reflect in my test files, resulting in two reflection registries and thus breaking reflection between my library and the test file. I am unable to test decorators from my library. I made a simple repo to illustrate the issue: https://github.com/dwieeb/ava-decorators

I had to use ava from master in order to debug tests in subprocesses and ultimately see that babel is wiping out the global Reflect and also substituting my polyfill for some core-js thing.

node 6 npm 3

relevant, unresolved: https://github.com/avajs/ava/issues/947

[novemberborn]

As of #1197 master no longer applies the runtime transform.

[imhoffd]

@lholznagel I did this, but I think const { Reflect } = global should work, too(?):

const g = <any>global;
const Reflect = g.Reflect;