Modify assertAndReturn to please TS v4.4.0 (unreleased)

Dtslint doesn't support not running against "next" [1], which means that
I can either temporarily remove type testing before publishing or fix
the method. The former would mean that the published version wouldn't
correspond to the tagged source code, which I don't enjoy.

[1] microsoft/dtslint#288

lib/assertions.ts

@@ -9,11 +9,11 @@ export function assert(value: any, message?: string): asserts value {
 }
 
 export function assertAndReturn<T>(
-  value: T | false | null | undefined,
+  value: T,
   message?: string
-): T {
+): Exclude<T, false | null | undefined> {
   assert(value, message);
-  return value;
+  return value as Exclude<T, false | null | undefined>;
 }
 
 export function assertIsString(