In JS, type annotations should not block errors from the rest of the program

[sandersn]

In a Javascript file that is checked by flow, we may see code like this (from create-react-app/packages/react-error-overlay/src/utils/parseCompileError.js)

export type ErrorLocation = {|
 fileName: string,
 lineNumber: number,
 colNumber?: number,
|}
function parseCompileError(message: string): ?ErrorLocation {
 const lines: Array<string> = message.split('\n');
 for (let i = 0; i < lines.length; i++) {
// ...................
 return fileName && lineNumber ? { fileName, lineNumber, colNumber } : null;
}

This behaves badly when the typescript compiler compiles it with checkJs on:

Expected behavior:

1. Errors on the type declaration and all the type annotations.
2. However, ErrorLocation should be declared as a type alias, and message, lines, etc should all have their declared types.
3. If they are used incorrectly, they should have errors.

Actual behavior:

1. Errors on the type declaration and all the type annotations.

In the language service:
2. message, lines, etc have the correct types, but ErrorLocation is type any.
3. If they are used incorrectly, they have errors.

In batch compilation:
2. No errors show up except those from (1), even if there are lots of other javascript files without type annotations.