Use `Array<T | U>` when dealing with union types instead of  `(T | U)[]`

#60565

We compile using the alwaysStrict flag so these directives are not needed

This part removes most `use strict` directives that are right after the file header

There are many places in the code that do `import Event, { ...} from '.../event'`.

This reverts commit 1828d8a7.

see https://basarat.gitbooks.io/typescript/content/docs/tips/nominalTyping.html and https://github.com/Microsoft/TypeScript/blob/7b48a182c05ea4dea81bab73ecbbe9e013a79e99/src/compiler/types.ts#L693-L698