Add option to indicate bundle encoding

Summary: public

To improve cold start performance we want to be able to avoid decoding the bundle at all. To make that happen we need to be able to generate a bundle encoded on `ucs2`. This diff adds support for indicating the encoding the Packager should use for bundling.

Reviewed By: davidaurelio

Differential Revision: D2582365

fb-gh-sync-id: 905384272a668910c57a1a2ca6d1b723c39233f8

local-cli/bundle/__tests__/saveBundleAndMap-test.js

@@ -41,10 +41,11 @@ describe('saveBundleAndMap', () => {
     saveBundleAndMap(
       codeWithMap,
       'ios',
-      bundleOutput
+      bundleOutput,
+      'utf8',
     );
 
-    expect(fs.writeFileSync.mock.calls[0]).toEqual([bundleOutput, code]);
+    expect(fs.writeFileSync.mock.calls[0]).toEqual([bundleOutput, code, 'utf8']);
   });
 
   it('should save sourcemaps if required so', () => {
@@ -55,6 +56,7 @@ describe('saveBundleAndMap', () => {
       codeWithMap,
       'ios',
       bundleOutput,
+      'utf8',
       sourceMapOutput
     );