Move preloaded modules to startup code section

Summary:We found that moving the preloaded modules to the startup section of the RAM Bundle improves TTI quite a bit by saving lots of through the bridge calls and injecting multiple modules at once on JSC. However, doing this on a non hacky way required a lot of work. The main changes this diff does are:
  - Add to `BundleBase` additional bundling options. This options are fetched based on the entry file we're building by invoking a module that exports a function (`getBundleOptionsModulePath`).
  - Implement `BundleOptions` module to include the `numPreloadedModules` attribute as a bundle additional option. This value is computed by getting the dependencies the entry file has and looking for the first module that exports a module we don't want to preload. The `numPreloadedModules` attribute is then used to decide where to splice the array of modules.
- Additional kung fu to make sure sourcemaps work for both preloaded and non preloaded modules.

Reviewed By: davidaurelio

Differential Revision: D3046534

fb-gh-sync-id: 80b676222ca3bb8b9eecc912a7963be94d3dee1a
shipit-source-id: 80b676222ca3bb8b9eecc912a7963be94d3dee1a

local-cli/bundle/output/unbundle/as-assets.js

@@ -34,7 +34,7 @@ function saveAsAssets(bundle, options, log) {
   } = options;
 
   log('start');
-  const {startupCode, startupModules, modules} = bundle.getUnbundle();
+  const {startupCode, startupModules, modules} = bundle.getUnbundle('ASSETS');
   log('finish');
 
   log('Writing bundle output to:', bundleOutput);

local-cli/bundle/output/unbundle/as-indexed-file.js

@@ -31,7 +31,7 @@ function saveAsIndexedFile(bundle, options, log) {
   } = options;
 
   log('start');
-  const {startupCode, modules} = bundle.getUnbundle();
+  const {startupCode, modules} = bundle.getUnbundle('INDEX');
   log('finish');
 
   log('Writing unbundle output to:', bundleOutput);
@@ -112,7 +112,7 @@ function buildModuleTable(buffers) {
       uInt32Buffer(currentLine),
     ]);
 
-    currentLine += linesCount - 1;
+    currentLine += linesCount;
 
     currentOffset += length;
     tableLength += entry.length;
@@ -129,7 +129,7 @@ function buildModuleBuffers(startupCode, modules, encoding) {
       modules.map(module =>
         moduleToBuffer(
           String(module.id),
-          module.code + '\n', // each module starts on a newline
+          module.code,
           encoding,
         )
       )

local-cli/bundle/output/unbundle/buildUnbundleSourcemap.js

@@ -24,12 +24,9 @@ const SourceMapConsumer = sourceMap.SourceMapConsumer;
  */
 function buildUnbundleSourcemap(bundle) {
   const generator = new sourceMap.SourceMapGenerator({});
-  const nonPolyfillModules = bundle.getModules().filter(module =>
-    !module.polyfill
-  );
 
-  let offset = 1;
-  nonPolyfillModules.forEach(module => {
+  let offset = 0;
+  bundle.getUnbundle('INDEX').allModules.forEach(module => {
     if (module.map) { // assets have no sourcemap
       const consumer = new SourceMapConsumer(module.map);
       consumer.eachMapping(mapping => {