When loading via AMD loader, module execution order can be unexpected

[sorvell]

This causes flakiness in ShadyCSS style processing because that library has load order requirements that users must adhere to.

1. a test loads polymer-legacy.js
2. polymer-legacy loads
   • legacy-element-mixin
     • legacy-element-mixin loads apply-shim as its first import
   • custom-style

Based on this, I expect apply-shim to load and execute before custom-style.

However, it does not (verified via logging and Edge element inspector):

[rictic]

I think what's happening is that we're not following the full es6 import ordering semantics. If you've got:

start.js

import 'foo';
import 'baz';

foo.js

import 'bar';

Then the execution order is defined as: bar, foo, baz, start, but I think our loader is racy and might run baz first.

So to do this right we can't just run module bodies when all of their imports are present, we also need to obey ordering of their parents' dependencies, which can be extra tricky when cycles get involved.

I'll put together some tests to verify that this is what's happening.