ES6 module support for "type-only import" like `goog.requireType` or TS `import type`

The type system currently has no way of referencing types in ES6 modules without importing them. We need some sort of weak reference / import type for ES6 modules.

Bike shedding ideas dump:

1. Allow paths in type names. Use : as a delimiter for path:type.

/** @param {./foo.js:Type.Nested} n */
function foo(n) {}

1. Some commented out import syntax that brings the names into the type scope.

// @import {Type} from './foo.js';

/** @param {Type.Nested} n */
function foo(n) {}

1. Since the option above might be difficult with the current way we handle JsDoc (needs to be attached to something) and because Type doesn’t really exist (it isn’t a variable), we could modify the above option to act more like how we handle typedefs, i.e. annotate a variable declaration.

/** @import {Type} from './foo.js'; */
let Type;

/** @param {Type.Nested} n */
function foo(n) {}

[ChadKillingsworth]

This isn’t restricted to ES6 modules. CommonJS need the same thing.

[evmar]

#1 resembles the analogous TS feature, I think. I’ve never used it myself but I think it was meant to look like dynamic imports, under the reasoning that imports in type position don’t have runtime implications.

var x: import('./y).Type; 

https://davidea.st/articles/typescript-2-9-import-types