When the --es-modules flag is provided import() will be used to load
all test files.

node 9.6 will not allow script files without .js extensions when --experimental-modules
flag is passed. I think node 9.7 will allow this so this commit can be reverted then.

Node versions less than 9.6 do not support dynamic es imports. Currently
using instanbul to calculate codecoverage calls some script without a .js file extension
which causes the tests to fail. Hopefully in v9.7 this will be
fixed.

Adds an integration test that uses cjs modules but is called with
the --es-modules flag.

Dynamic imports are async and therefore errors need to be explicitly handled.

This reverts commit 10736aa.

This reverts commit b28d2db.