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.dart

@@ -6,9 +6,10 @@ export './src/core/util.dart';
 export 'package:angular2/src/facade/lang.dart' show enableProdMode;
 export './src/core/di.dart' hide ForwardRefFn, resolveForwardRef, forwardRef;
 export './src/facade/facade.dart';
-export './src/core/application_ref.dart' show platform, createNgZone, PlatformRef, ApplicationRef;
+export './src/core/application_ref.dart' show createPlatform, assertPlatform,
+    disposePlatform, getPlatform,
+    coreLoadAndBootstrap, coreBootstrap, createNgZone, PlatformRef, ApplicationRef;
 export './src/core/application_tokens.dart' show APP_ID,
-  APP_COMPONENT,
   APP_INITIALIZER,
   PACKAGE_ROOT_URL,
   PLATFORM_INITIALIZER;