fix($compile): throw an exception when an attribute is unassigned

[panuhorsmalahti]

Documentation states that NON_ASSIGNABLE_MODEL_EXPRESSION exception should be thrown if
a two-way attribute doesn't exist. This commit adds a simple test and throws an error
if the property is missing. Note that the documentation should probably be updated
and a different exception should be thrown using minErr.

This commit refers to issue #6060

[caitp]

I realize that this is more of a match for what the docs say, but the trouble with this is that it is actually a breaking change, as many directives with isolate scopes are not making use of the "?" optional symbol, and simply ignore an attribute if it's undefined.

I don't think we're going to be able to merge this fix until 1.3 in a few weeks

[panuhorsmalahti]

Okay. Should I work on the documentation and create a new error page for this?

[Narretz]

I think this should be merged in some way before 1.3 is released.

[cironunes]

You don't need the else statement since the `ìf`` returns.

So that you could do:

if (optional) return;
throw new Error(...);

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[cironunes]

@panuhorsmalahti could you squash commits into the first one? Can you also add unit tests for this fix?

[panuhorsmalahti]

@googlebot, I wonder why that is. The email in the commit was '[email protected]', which is in the CLA. I added my GitHub account name to the CLA now.

[googlebot]

CLAs look good, thanks!

[panuhorsmalahti]

Squashed the commits and synced my fork.

[petebacondarwin]

We prefer to use minErr when throwing errors... we may need to create a new error code.

throw $compileMinErr('reqattr',
                      "Attribute '{0}' used with directive '{1}' is not optional!",
                      attrName, newIsolateScopeDirective.name);

[petebacondarwin]

@panuhorsmalahti Could you update to use $compileMinErr?

We will need a new error info document in https://github.com/angular/angular.js/tree/master/docs/content/error/%24compile explaining what this error means and how to fix it.

Also the PR will need a unit test for $compile similar to https://github.com/angular/angular.js/blob/master/test/ng/compileSpec.js#L3482

[IgorMinar]

During the fixit#1 a patch was created by @vlad (I believe) to address this during compilation not linking and do it for all isolate scope mapping modes (=,&,@). That approach is preferable to this one.

@vlad, I don't see your PR with this change. can you send it to us and mention that it's part of fixit please?

[Narretz]

Better PR here: #16129