ES6 module support breaks for paths containing some chars (eg `+`)

[alexeagle]

Here's a smallish repro:
https://github.com/FrozenPandaz/closure-compiler-angular-bundling/tree/plus_directories

If one of the files in the compilation uses ES6 syntax and a + in the name, we get

built/+basic.js:1: ERROR - "module$built$+basic" is not a valid ECMASCRIPT6 qualif
ied name

this is a problem because the Angular style guide recommends using a + prefix on directories containing lazy routes.

[ChadKillingsworth]

That's a trivial fix.

[alexeagle]

I can send a PR for this. I think the fix belongs here:
https://github.com/google/closure-compiler/blob/master/src/com/google/javascript/jscomp/deps/ModuleNames.java#L64
any idea where is the right place to add the test?

[MatrixFrog]

https://github.com/google/closure-compiler/tree/master/test/com/google/javascript/jscomp/runtime_tests/module_tests or https://github.com/google/closure-compiler/blob/master/test/com/google/javascript/jscomp/ProcessEs6ModulesTest.java I would guess.

[ChadKillingsworth]

Right here: https://github.com/google/closure-compiler/blob/master/src/com/google/javascript/jscomp/deps/ModuleNames.java#L45-L69

[alexeagle]

Mailed a google-internal change for it (I can also post a PR if you'd like to review, Chad. I didn't see how to run the runtime_tests without fighting with my IDE setup and Maven...)

[MatrixFrog]

We don't have a way to run the runtime tests externally, currently.

[ChadKillingsworth]

Yeah I can't review internal PRs and it's much easier for you to do internally. If it breaks something, I'll just PR the fix and have you review it 😄