Suppress missing require from externs (reworked)

[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

This is a rework of PR #1192 which follows the behaviour of the rest of the class more closely. The check now happens before adding to usages, in visitClassNode, rather than in visitScriptNode, as per @MatrixFrog comment #1192 (comment)

[supersteves]

@blickly wrote in #1192 (comment)

Looks live visitNewNode doesn't call maybeAddUsage

@MatrixFrog wrote in #1192 (comment)

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

In fact because this is an ES6 class, the problem was in visitClassNode, which was unreservedly adding a usage for the extends clause.

Line 406 no longer exists, but I think this was maybeAddUsage which is specific to types mentioned in JSDoc, and not the extends clause.

This fix was done sympathetically to visitNewNode; the unit test is unchanged from the original PR.

[ChadKillingsworth]

This makes sense. I'm surprised I haven't hit this yet.

[MatrixFrog]

nit: Make the code structure follow this comment with an early return:

if (!extendClass.isQualifiedName()) {
return;
}

// Grab the root superclass namespace.
Node root = ...

In fact then we can probably drop the comment too.

[MatrixFrog]

lgtm other than that. Thank you.

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[googlebot]

CLAs look good, thanks!

[supersteves]

@MatrixFrog Nits done, thanks

[supersteves]

Thanks!