Suppress missing require from externs

[supersteves]

This is to fix the case where you have React defined globally in externs:

// Externs:

var React = {};

/**
 * @constructor
 * @param {Object} props
 */
React.Component = function(props) {};

// Main code:

class X extends React.Component {
}

Without this PR you get the following warning, which is incorrect since externs don't need to be required as they are global:

'React.Component' used but not goog.require'd

If this is accepted, PR #1147 for suppressing missing requires on class nodes can be deleted.

[blickly]

nit: spaces around the ==

[MatrixFrog]

I thought the existing var.isExtern() check on line 406 was supposed to make this work. We should figure out why it's not.

[blickly]

Looks live visitNewNode doesn't call maybeAddUsage

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[googlebot]

CLAs look good, thanks!

[supersteves]

I'm afraid I broke this PR when I rebased and resolved conflicts with a formatting change. I've committed a fix.

[supersteves]

Closing, reworked here:
#1366