For the below code:

import {Subscription} from 'rxjs/Subscription';
class Foo {
    sayHi(name) {
        return `Hi ${name}, welcome!`;
    }
}
export let sqr = x => x * x;
export default Subscription;

If `rxjs/Subscription' import does not resolve, then the emit is:

"use strict";
var Foo = (function () {
    function Foo() {
    }
    Foo.prototype.sayHi = function (name) {
        return "Hi " + name + ", welcome!";
    };
    return Foo;
}());
exports.sqr = function (x) { return x * x; };

Notice that the import and export default has been elided.

This can be resolved by setting isolatedModules, or by referencing a property on Subscription (even if that could be a type on a namespace also - e.g. export default Subscription.IFoo).

This is especially an issue for Salsa when the input is JavaScript rather than TypeScript, as there are no errors/warning for unresolved modules/types (and the reference can't be to a type).

Spoke with Mohamed, and he agrees this should be fixed (and should be easy).