Implicit Constructor proposal

Solution for https://github.com/dart-lang/language/issues/107, [Related docs](https://github.com/dart-lang/language/blob/master/working/0107%20-%20implicit-constructors/)

This issue is for discussing the pros and cons of this particular proposal.
General discussion about the issue should go in #107 where everybody can see it.

I propose to add a new keyword – `implicit` that can be added to constructors and factories.

Read issue #107 and the related proposal in working/0107 - implicit-constructors/ before assessing the proposed `implicit` keyword. This issue is for weighing pros and cons, so done means reaching and documenting a decision on the proposal rather than making a localized code change.

Written by the indexing model from the issue text.