[FalconJX] Intermittently adds incorrect goog.require() for class in external library

[Josh Tynjala]

Recently, I've noticed that the output of asjsc is not always identical.
Sometimes, it will incorrectly add a goog.require() for a class that is in
the external library path. Not always, though.

Here's some simplified example output (comment added for emphasis):

goog.provide('feathers.core.FeathersControl');

goog.require('createjs.Container'); //this should not be here

/**
 * Constructor.
 * @constructor
 * @extends {createjs.Container}
 */
feathers.core.FeathersControl = function() {
};

Here's what I've discovered so far:

1) In org.apache.flex.compiler.internal.projects.FlexJSProject, on line
170, the call to linkageChecker.isExternal(cu) inside the
isExternalLinkage() method sometimes returns false for createjs.Container.
It should always return true.

2) Earlier in the same method, the linkageChecker is created, if it is null:

if (linkageChecker == null)
{
    ts = getTargetSettings();
    linkageChecker = new LinkageChecker(this, ts);
}

I tried to add System.out.println() calls before and after the
LinkageChecker instance is created:

if (linkageChecker == null)
{
    ts = getTargetSettings();
    System.out.println("Creating new LinkageChecker");
    linkageChecker = new LinkageChecker(this, ts);
    System.out.println("Created LinkageChecker");
}

This is my output when the issue reproduces:

Creating new LinkageChecker
Creating new LinkageChecker
Created LinkageChecker
Created LinkageChecker

When it works correctly, only one LinkageChecker instance is created:

Creating new LinkageChecker
Created LinkageChecker

It appears that isExternalLinkage() may get called again before the
linkageChecker variable is set. I don't see anything in the LinkageChecker
constructor that might cause that. It's just setting a couple of member
variables.

Is the compiler using more than one thread? Could different threads be
calling isExternalLinkage(), and am I seeing a race condition?

- Josh