The globally named object, may be any valid property key (string, number or symbol)
Optional useCached: boolean[Optional] used for testing to bypass the cached lookup, when true this will
cause the cached global to be reset.
// This does not cause the evaluation to occur
window.myGlobal = "Hello";
let cachedValue = getInst("myGlobal");
// cachedValue === "Hello"
window.myGlobal = "Darkness";
// getInst("myGlobal") === "Darkness"
let promiseCls = getInst("Promise");
// May throw if the global is not supported by the runtime
// otherwise the Promise class.
Return the named global object if available, will return null if the object is not available.