How to avoid "unresolved function" error with TypeScript mixins?

WebStorm is reporting an unresolved function when I use Durandal's Events.includeIn() mixin method on a TypeScript class:

class Foo {
  bar() {
    ...
    this.trigger('myEvent', payload);  // trigger is unresolved function
    ...
  }
}

Events.includeIn(Foo);

Is there a way to resolve this without using WebStorm to suppress each unresolved call?