extern definition for measureText should return non-null

In the extern html5.js the return type should be non-null. Add an exclamation point like this:

/**
 * @param {string} text
 * @return {!TextMetrics}
 * @nosideeffects
 */
CanvasRenderingContext2D.prototype.measureText = function(text) {};

The spec for measureText makes no mention of possibly returning null.

This change helps avoid needing code like this:

var textMetrics = context.measureText(s);
if (!textMetrics) {
 throw new Error();
}
var textWidth = textMetrics.width;

instead you can write

var textWidth = context.measureText(s).width;

[myphysicslab]

After thinking it over, I’ll modify the pull request to only change return values to be non-nullable, not parameter values. The question about non-nullable parameter values should be done on a separate issue and pull request.

[myphysicslab]

Fixed by #2435