Transforms.preloadIcrfFixed resolving too soon

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/7xyQSQJSHk8

The function below prints out `FAILURE` the first time it runs and `SUCCESS` subsequent times you press the button. I was under the impression that if you waited for the `preloadIcrfFixed` promise to resolve, calls to `computeIcrfToFixedMatrix` should work.

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer');

Sandcastle.addDefaultToolbarButton('Default styling', function() {

var start_time = new Date(1285216305688);
var stop_time = new Date(1285259505688);

var timeInterval = new Cesium.TimeInterval({
start : start_time,
stop : stop_time,
isStartIncluded : true,
isStopIncluded : true
});

var promise = Cesium.Transforms.preloadIcrfFixed(timeInterval);

promise.then(function() {

var pointInFixed = Cesium.Cartesian3.fromDegrees(0.0, 0.0);
var fixedToIcrf = Cesium.Transforms.computeIcrfToFixedMatrix(stop_time);
var pointInInertial = new Cesium.Cartesian3();
if (Cesium.defined(fixedToIcrf)) {
pointInInertial = Cesium.Matrix3.multiplyByVector(fixedToIcrf, pointInFixed, pointInInertial);
console.log("SUCCESS");
} else{
console.log("FAILURE");
}
});
});
```

Start by reproducing the Sandcastle example using Transforms.preloadIcrfFixed and computeIcrfToFixedMatrix with the supplied TimeInterval. Trace when the preload promise resolves relative to matrix availability; done means the first promise callback produces a defined matrix and the documented computation succeeds without requiring a second button press.

Written by the indexing model from the issue text.