Treat Dart maps like maps, not (JavaScript) objects with field access

[chalin]

This seems like a carry over from AngularJS: maps can be treated as objects, in particular, (simulated) field access is currently supported:

• map.key is treated as a synonym for map[key]

and this for both reading and writing (as well as creating) map "fields".

While this is natural in JavaScript, it is at odds with Dart's clean separation of objects/classes from the Map datatype. IMHO, in designing AngularDart, every effort should be made to keep to a minimum differences in syntax and semantics between the AngularDart expression/statement language and the Dart language.

This JavaScript-ism is what prevented Map properties from being accessible, leading to, e.g., #397. While #605 offers a solution, it would not have been required in the first place if maps had been treated solely as maps (and hence without field access). I believe that a (full) proper solution is to remove the AngularJS "map field access" feature from AngularDart.

[chalin]

@mhevery comments? (FYI, if/when you agree, I would not mind attempting to address this now that I have spent the time to study that part of the code.)

[mhevery]

See: #397 (comment)

[chalin]

Subsumed by issues covered by #397.