build: consolidate tsc to ease migration to @types/ based typings delivery

I actually tried to use @types/* directly but came across several issues which prevented me
from switching over:
- https://github.com/Microsoft/TypeScript/issues/8715
- https://github.com/Microsoft/TypeScript/issues/8723

tools/broccoli/multi_copy.ts

@@ -1,5 +1,3 @@
-/// <reference path="../typings/node/node.d.ts" />
-/// <reference path="../typings/fs-extra/fs-extra.d.ts" />
 /// <reference path="./broccoli-writer.d.ts" />
 
 import Writer = require('broccoli-writer');