Buffer.from(): avoid clash between Iterable<number> and string case for first arg
Buffer.from(): avoid clash between Iterable<number> and string case for first arg

Summary:
In some cases, Flow seems to eagerly choose the Iterable<number> case for the first argument to Buffer.from(), reporting the following error when Buffer.from(aString, encoding) is used:

```
inconsistent use of library definitions
274:     @iterator(): Iterator<string>;
                                ^^^^^^ string. This type is incompatible with. See lib: /private/tmp/flow/flowlib_253af11f/core.js:274
111:   static from(value: Iterable<number>): this;
                                   ^^^^^^ number. See lib: /private/tmp/flow/flowlib_253af11f/node.js:111
```

Reordering the lib def for Buffer.from() fixes that problem for now, though the over-eagerness in choosing the Iterable<number> case should obviously be investigated too.
Closes https://github.com/facebook/flow/pull/3225

Differential Revision: D4441607

Pulled By: gabelevi

fbshipit-source-id: 85ebe32476a4754b3c306952b468e730e8acd13a

gabro/flow,nmote/flow,claudiopro/flow,samwgoldman/flow,JonathanUsername/flow,samwgoldman/flow,facebook/flow,gabro/flow,gabro/flow,ylu1317/flow,gabelevi/flow,popham/flow,gabro/flow,nmote/flow,JonathanUsername/flow,mroch/flow,mroch/flow,TiddoLangerak/flow,JonathanUsername/flow,gabelevi/flow,mroch/flow,ylu1317/flow,claudiopro/flow,gabelevi/flow,facebook/flow,popham/flow,facebook/flow,MichaelDeBoey/flow,jamesgpearce/flow,fletcherw/flow,jamesgpearce/flow,facebook/flow,samwgoldman/flow,nmote/flow,TiddoLangerak/flow,MichaelDeBoey/flow,popham/flow,popham/flow,TiddoLangerak/flow,gabelevi/flow,facebook/flow,AgentME/flow,TiddoLangerak/flow,TiddoLangerak/flow,JonathanUsername/flow,AgentME/flow,ylu1317/flow,gabelevi/flow,mroch/flow,ylu1317/flow,samwgoldman/flow,popham/flow,jamesgpearce/flow,AgentME/flow,JonathanUsername/flow,facebook/flow,JonathanUsername/flow,fletcherw/flow,nmote/flow,gabro/flow,fletcherw/flow,fletcherw/flow,mroch/flow,AgentME/flow,claudiopro/flow,jamesgpearce/flow,jamesgpearce/flow,fletcherw/flow,gabelevi/flow,claudiopro/flow,mroch/flow,mroch/flow,samwgoldman/flow,nmote/flow,jamesgpearce/flow,samwgoldman/flow,samwgoldman/flow,MichaelDeBoey/flow,nmote/flow,MichaelDeBoey/flow,jamesgpearce/flow,gabelevi/flow,facebook/flow,MichaelDeBoey/flow,popham/flow,claudiopro/flow,gabro/flow,nmote/flow,ylu1317/flow,fletcherw/flow,AgentME/flow,JonathanUsername/flow,popham/flow