Before evaluating a module, all variables declared at the top-level
in _any_ of the modules in the dependency graph must be initialized.
This is observable because a module A can access a variable imported
from module B (e.g. a function) at a point when module B's body hasn't
been evaluated yet.

We achieve this by implementing modules internally as generators with
two states (not initialized, initialized).

R=adamk@chromium.org
BUG=v8:1569

Review-Url: https://codereview.chromium.org/2375793002
Cr-Commit-Position: refs/heads/master@{#39871}