Revert "feat(core): extract platforms out of core"

This reverts commit 3f4628c0b0.

modules/angular2/angular2.ts

@@ -2,5 +2,5 @@ export * from './common';
 export * from './core';
 export * from './profile';
 export * from './lifecycle_hooks';
-export * from './platform/browser';
+export * from './bootstrap';
 export * from './upgrade';