Report error if code precedes /// reference

a.ts:

'use strict'
/// <reference path='b.d.ts'/>

var x: Foo;

b.d.ts:

interface Foo { x: string; }
declare module "b" {
        export = Foo;
}

D:\test\refs\a> tsc a.ts
a.ts(4,8): error TS2304: Cannot find name 'Foo'.

There's no mention in section 11.1.1 of the spec that /// references must be the first thing in a file in order to work, although I do remember us deciding that was the case at one point. If that is the rule we should update the spec appropriately. We should also add an error to the compiler or else you may spend awhile trying to figure this out since we don't provide much (read: anything) in the way of diagnostics for debugging reference resolution.