Originally reported in#8. Not passing thenoEmitHelpers TypeScript compiler option generates multiple__extends functions which supersede the "runtime"__extends implementation when bundled.

Ideally third party plugins should compile withnoEmitHelpers enabled, but the reality is that many of them don't do so.@NathanaelA has created a webpack loader that strips the module-local__extends code, so that apps work without modifying third party plugins. If given permission, we should publish and document this loader.