refactor(core): support non reflective bootstrap.

This changes Angular so that it can be used without reflection (assuming a codegen for injectors).

BREAKIKNG CHANGE:
- Drops `APP_COMPONENT` provider. Instead, inject
  `ApplicationRef` and read its `componentTypes` property.
- long form bootstrap has changed into the following:
  ```
  var platform = createPlatform(ReflectiveInjector.resolveAndCreate(BROWSER_PROVIDERS));
  var appInjector =
    ReflectiveInjector.resolveAndCreate([BROWSER_APP_PROVIDERS, appProviders], platform.injector);
  coreLoadAndBootstrap(appInjector, MyApp);
  ```

modules/angular2/core.ts

@@ -9,10 +9,19 @@ export * from './src/core/prod_mode';
 export * from './src/core/di';
 export * from './src/facade/facade';
 export {enableProdMode} from 'angular2/src/facade/lang';
-export {platform, createNgZone, PlatformRef, ApplicationRef} from './src/core/application_ref';
+export {
+  createPlatform,
+  assertPlatform,
+  disposePlatform,
+  getPlatform,
+  coreBootstrap,
+  coreLoadAndBootstrap,
+  createNgZone,
+  PlatformRef,
+  ApplicationRef
+} from './src/core/application_ref';
 export {
   APP_ID,
-  APP_COMPONENT,
   APP_INITIALIZER,
   PACKAGE_ROOT_URL,
   PLATFORM_INITIALIZER