Currently, if you create an attribute with both a setter _and_ an
initial value, the compiler will initially set the attribute to
`null`, and then call the setter with the initial value. This is not
actually right, but it lets setters get away with code like:
<attribute name="foo" value="null" />
<setter name="foo" args="newfoo">
if (this.foo !== newfoo) {
...
}
</setter>
People sometimes write this as an optimization, to avoid a complex
setter if the value is not actually changing.
I propose that the 'initial initial' value of an attribute like this
be `undefined`, rather than `null`, because it really has not been set
yet. Furthermore, in an optimization like the above, the setter code
will not be called on the `null` initial value of the attribute
(because the compiler has already set it to `null`). This can lead to
subtle bugs.
Pro:
The setter code for cases like the above will not be erroneously
skipped if the initial value is `null`.
Con:
The setter code must take care checking the value of the attribute.
Since it will initially be `undefined`, referencing it as `this.foo`
will generate a warning in the swf8 runtime. To avoid the warning,
the code must be re-written as `this['foo']`.
Comments? Questions?
