[react-packager] Cache in which bundle is each module

Summary:
Not that at the moment a module can be present in multiple bundles, so the new API will return only one of them. In the near future we'll impose the invariant that a module can only be present in a single bundle so this API will return the exact bundle in which it is.

packager/react-packager/src/BundlesLayout/__tests__/BundlesLayout-test.js

@@ -146,5 +146,42 @@ describe('BundlesLayout', () => {
         ])
       );
     });
+
+    pit('separate cache in which bundle is each dependency', () => {
+      DependencyResolver.prototype.getDependencies.mockImpl((path) => {
+        switch (path) {
+          case '/root/index.js':
+            return Promise.resolve({
+              dependencies: [dep('/root/index.js'), dep('/root/a.js')],
+              asyncDependencies: [['/root/b.js']],
+            });
+          case '/root/a.js':
+            return Promise.resolve({
+              dependencies: [dep('/root/a.js')],
+              asyncDependencies: [],
+            });
+          case '/root/b.js':
+            return Promise.resolve({
+              dependencies: [dep('/root/b.js')],
+              asyncDependencies: [['/root/c.js']],
+            });
+          case '/root/c.js':
+            return Promise.resolve({
+              dependencies: [dep('/root/c.js')],
+              asyncDependencies: [],
+            });
+          default:
+            throw 'Undefined path: ' + path;
+        }
+      });
+
+      var layout = newBundlesLayout();
+      return layout.generateLayout(['/root/index.js']).then(() => {
+        expect(layout.getBundleIDForModule('/root/index.js')).toBe(0);
+        expect(layout.getBundleIDForModule('/root/a.js')).toBe(0);
+        expect(layout.getBundleIDForModule('/root/b.js')).toBe(1);
+        expect(layout.getBundleIDForModule('/root/c.js')).toBe(2);
+      });
+    });
   });
 });

packager/react-packager/src/BundlesLayout/index.js

@@ -26,6 +26,8 @@ class BundlesLayout {
   constructor(options) {
     const opts = validateOpts(options);
     this._resolver = opts.dependencyResolver;
+
+    this._moduleToBundle = Object.create(null);
   }
 
   generateLayout(entryPaths, isDev) {
@@ -44,9 +46,10 @@ class BundlesLayout {
           .then(modulesDeps => {
             let syncDependencies = Object.create(null);
             modulesDeps.forEach(moduleDeps => {
-              moduleDeps.dependencies.forEach(dep =>
+              moduleDeps.dependencies.forEach(dep => {
                 syncDependencies[dep.path] = dep
-              );
+                this._moduleToBundle[dep.path] = bundles.length;
+              });
               pending = pending.concat(moduleDeps.asyncDependencies);
             });
 
@@ -60,6 +63,10 @@ class BundlesLayout {
       },
     );
   }
+
+  getBundleIDForModule(path) {
+    return this._moduleToBundle[path];
+  }
 }
 
 // Runs the body Promise meanwhile the condition callback is satisfied.