Invoking undefined as a function throws now exception.

In the following code, `this.bar` is `undefined`. Yet invoking it as a function doesn't make Oh Noes appear.
Oh Noes should appear as soon as you click the mouse button.

```
var Boo = function() {
};

Boo.prototype.foo = function() {
this.bar();
};

var boo = new Boo();

mouseClicked = function() {
boo.foo();
};
```

Reproduce the provided JavaScript snippet in the live editor and click the mouse button to observe the current behavior. Trace the runtime error handling from the undefined function call, then verify that the expected “Oh Noes” message appears after the click without changing the example’s intended behavior.

Written by the indexing model from the issue text.