Add infra for Prepack build option

Summary: This adds a build option for using Prepack (an experimental packager) to
build a bundle. It doesn't actually take on the npm package dependency
because it's not published/open source (yet).

This will be used while we experiment and should be maintained as the
build system changes so that we can continue getting fresh builds.

I found that saveBundleAndMap and processBundle were over abstracted and
got in my way so I inlined it and removed the unit tests because the unit
test was testing trivial code that is likely to change interface.

I went with a separate build phase and a separate Bundle class even though
there are a lot of commonalities. I imagine that the requirements for
Prepack will continue to diverge. Especially for source maps but a larger
refactor could try to unify these a bit more. The fact that modules are
wrapped before the write phase seems to be an unfortunate architecture
that makes this difficult.
Closes https://github.com/facebook/react-native/pull/4226

Reviewed By: amasad

Differential Revision: D2673760

Pulled By: sebmarkbage

fb-gh-sync-id: 299ccc42e4be1d9dee19ade443ea3388db2e39a8

local-cli/bundle/bundleCommandLineArgs.js

@@ -27,6 +27,14 @@ module.exports = [
     command: 'dev',
     description: 'If false, warnings are disabled and the bundle is minified',
     default: true,
+  }, {
+    command: 'prepack',
+    description: 'If true, the output bundle will use the Prepack format.',
+    default: false
+  }, {
+    command: 'bridge-config',
+    description: 'File name of a a JSON export of __fbBatchedBridgeConfig. Used by Prepack. Ex. ./bridgeconfig.json',
+    type: 'string'
   }, {
     command: 'bundle-output',
     description: 'File name where to store the resulting bundle, ex. /tmp/groups.bundle',