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/__tests__/saveBundleAndMap-test.js

@@ -1,63 +0,0 @@
-/**
- * Copyright (c) 2015-present, Facebook, Inc.
- * All rights reserved.
- *
- * This source code is licensed under the BSD-style license found in the
- * LICENSE file in the root directory of this source tree. An additional grant
- * of patent rights can be found in the PATENTS file in the same directory.
- */
-'use strict';
-
-jest.autoMockOff();
-
-jest.mock('fs');
-jest.mock('../sign');
-
-const saveBundleAndMap = require('../saveBundleAndMap');
-const fs = require('fs');
-const temp = require('temp');
-
-const code = 'const foo = "bar";';
-const map = JSON.stringify({
-  version: 3,
-  file: 'foo.js.map',
-  sources: ['foo.js'],
-  sourceRoot: '/',
-  names: ['bar'],
-  mappings: 'AAA0B,kBAAhBA,QAAOC,SACjBD,OAAOC,OAAO'
-});
-
-describe('saveBundleAndMap', () => {
-  beforeEach(() => {
-    fs.writeFileSync = jest.genMockFn();
-  });
-
-  it('should save bundle', () => {
-    const codeWithMap = {code: code};
-    const bundleOutput = temp.path({suffix: '.bundle'});
-
-    saveBundleAndMap(
-      codeWithMap,
-      'ios',
-      bundleOutput,
-      'utf8',
-    );
-
-    expect(fs.writeFileSync.mock.calls[0]).toEqual([bundleOutput, code, 'utf8']);
-  });
-
-  it('should save sourcemaps if required so', () => {
-    const codeWithMap = {code: code, map: map};
-    const bundleOutput = temp.path({suffix: '.bundle'});
-    const sourceMapOutput = temp.path({suffix: '.map'});
-    saveBundleAndMap(
-      codeWithMap,
-      'ios',
-      bundleOutput,
-      'utf8',
-      sourceMapOutput
-    );
-
-    expect(fs.writeFileSync.mock.calls[1]).toEqual([sourceMapOutput, map]);
-  });
-});