refactor(di): simplify Injector API

modules/di/src/injector.js

@@ -30,19 +30,11 @@ export class Injector {
   }
 
   get(token) {
-    return this.getByKey(Key.get(token));
+    return this._getByKey(Key.get(token), false, false);
   }
 
   asyncGet(token) {
-    return this.asyncGetByKey(Key.get(token));
-  }
-
-  getByKey(key:Key) {
-    return this._getByKey(key, false, false);
-  }
-
-  asyncGetByKey(key:Key) {
-    return this._getByKey(key, true, false);
+    return this._getByKey(Key.get(token), true, false);
   }
 
   createChild(bindings:List):Injector {

modules/di/src/key.js

@@ -13,6 +13,8 @@ export class Key {
   }
 
   static get(token) {
+    if (token instanceof Key) return token;
+
     if (MapWrapper.contains(_allKeys, token)) {
       return MapWrapper.get(_allKeys, token)
     }

modules/di/test/di/key_spec.js

@@ -10,5 +10,9 @@ export function main() {
     it('should not be equal to another key if types are different', function () {
       expect(Key.get('car')).not.toBe(Key.get('porsche'));
     });
+
+    it('should return the passed in key', function () {
+      expect(Key.get(Key.get('car'))).toBe(Key.get('car'));
+    });
   });
 }