Remove reference to WeakMap from tslib.d.ts

[rbuckton]

Unfortunately, WeakMap may not be declared when using tslib with a project that has "lib": ["es5"] set.

Fixes: #93

[DanielRosenwasser]

Honestly I wonder if we should just make them any. Even that constraint of object is limiting us to 2.1. But let's just get this in.

[rbuckton]

I don't see that as an issue since 2.1 is also the first version of TypeScript that supported tslib/--importHelpers: http://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html#support-for-external-helpers-library-tslib

[ExE-Boss]

Would adding the following solve this?

/// <reference lib="es2015.collection"/>

Or would it cause unintended consequences?

[HolgerJeromin]

Would adding the following solve this?
/// <reference lib="es2015.collection"/>
Or would it cause unintended consequences?

This would pull the collection in every consumer of tslib.d.ts, would it?

[DanielRosenwasser]

/// <reference lib /> doesn't work for older versions. Honestly, tslib isn't meant to be used directly, so even having these types be accurate isn't useful.