Specify new strong mode instantiate to bound rules

[munificent]

This is the language spec issue for #27526.

[dgrove]

@floitschG can this be assigned to the right person?

[eernstg]

We have a feature spec for this: instantiate-to-bound.md. It still remains to integrate this material into the language specification.

[eernstg]

No milestone now: Other issues of a similar nature have a milestone pertaining to the feature specification, but instantiate-to-bound.md has been landed and is not expected to need updates.

The remaining work is to integrate it into the language specification, as mentioned in the previous comment, but that does not block Dart 2.

[eernstg]

Closing: This feature has been made part of dartLangSpec.tex, with d118c7f as the last commit.