Add --maxWorkers flag and allow transformers to run in-band.

Summary:
This diff cleans up some cruft and adds some features:

* It removes the usage of an env variable to control workers.
* It removes the lazy and handwavy calculation on how many workers to use for jest-haste-map. Jest itself uses the maximum amount of workers available and it has never been reported as an issue – especially since it is a one-time startup cost of about 3 seconds on a cold cache only.
* It adds a `--max-workers` flag to replace the env variable. This one is able to control both the number of workers for `jest-haste-map` as well as the transformers.
* It makes the transformers run in the parent process if 1 or fewer workers are are specified. This should help with debugging.

Once you approve this diff, I will publish a new version of metro to npm and update the version used in RN and remove the use of the env variable altogether: https://our.intern.facebook.com/intern/biggrep/?corpus=xplat&filename=&case=false&view=default&extre=&s=REACT_NATIVE_MAX_WORKERS&engine=apr_strmatch&context=false&filter[uninteresting]=false&filter[intern]=false&filter[test]=false&grep_regex=

Note: the process of adding a CLI option is really broken. Commander also has a weird API. We should consider building a better public API for Metro and then consider how to build a new CLI on top of it and simplify our internal integration. I really don't like how Metro is integrated across pieces of the RN cli in ways that is hard to manage. But that is a larger task for another time :)

Reviewed By: jeanlauliac

Differential Revision: D5217726

fbshipit-source-id: 74efddbb87755a9e744c816fbc62efa21f6a79bf

local-cli/bundle/buildBundle.js

@@ -13,7 +13,7 @@
 
 const log = require('../util/log').out('bundle');
 const Server = require('metro-bundler/build/Server');
-const Terminal = require('metro-bundler/build/lib/TerminalClass');
+const Terminal = require('metro-bundler/build/lib/Terminal');
 const TerminalReporter = require('metro-bundler/build/lib/TerminalReporter');
 const TransformCaching = require('metro-bundler/build/lib/TransformCaching');
 
@@ -38,6 +38,7 @@ function buildBundle(
   args: OutputOptions & {
     assetsDest: mixed,
     entryFile: string,
+    maxWorkers: number,
     resetCache: boolean,
     transformer: string,
   },
@@ -89,6 +90,7 @@ function buildBundle(
       getTransformOptions: config.getTransformOptions,
       globalTransformCache: null,
       hasteImpl: config.hasteImpl,
+      maxWorkers: args.maxWorkers,
       platforms: defaultPlatforms.concat(platforms),
       postMinifyProcess: config.postMinifyProcess,
       postProcessModules: config.postProcessModules,

local-cli/bundle/bundleCommandLineArgs.js

@@ -31,6 +31,12 @@ module.exports = [
     command: '--bundle-encoding [string]',
     description: 'Encoding the bundle should be written in (https://nodejs.org/api/buffer.html#buffer_buffer).',
     default: 'utf8',
+  }, {
+    command: '--max-workers [number]',
+    description: 'Specifies the maximum number of workers the worker-pool ' +
+      'will spawn for transforming files. This defaults to the number of the ' +
+      'cores available on your machine.',
+    parse: (workers: string) => Number(workers),
   }, {
     command: '--sourcemap-output [string]',
     description: 'File name where to store the sourcemap file for resulting bundle, ex. /tmp/groups.map',