refactor(core): separate reflective injector from Injector interface

BREAKING CHANGE:
- Injector was renamed into `ReflectiveInjector`,
  as `Injector` is only an abstract class with one method on it
- `Injector.getOptional()` was changed into `Injector.get(token, notFoundValue)`
  to make implementing injectors simpler
- `ViewContainerRef.createComponent` now takes an `Injector`
  instead of `ResolvedProviders`. If a reflective injector
  should be used, create one before calling this method.
  (e.g. via `ReflectiveInjector.resolveAndCreate(…)`.

modules/angular2/test/router/location/location_spec.ts

@@ -12,7 +12,7 @@ import {
   SpyObject
 } from 'angular2/testing_internal';
 
-import {Injector, provide} from 'angular2/core';
+import {Injector, provide, ReflectiveInjector} from 'angular2/core';
 import {CONST_EXPR} from 'angular2/src/facade/lang';
 
 import {Location, LocationStrategy, APP_BASE_HREF} from 'angular2/platform/common';
@@ -26,7 +26,7 @@ export function main() {
     function makeLocation(baseHref: string = '/my/app', provider: any = CONST_EXPR([])): Location {
       locationStrategy = new MockLocationStrategy();
       locationStrategy.internalBaseHref = baseHref;
-      let injector = Injector.resolveAndCreate(
+      let injector = ReflectiveInjector.resolveAndCreate(
           [Location, provide(LocationStrategy, {useValue: locationStrategy}), provider]);
       return location = injector.get(Location);
     }