Duplicate "Add type annotation" assist

[scheglov]

[FMorschel]

I also tested this with:

var map = {};

The output is:

var map <, >{};

With the cursor at the same position. Which I'm unsure is what we want since this could also be a set. I'll leave it to the team to decide since I don't have any strong opinions here.

[bwilkerson]

We should not duplicate the type arguments in the edit.

We shouldn't add a list of type arguments whose names are missing.

We shouldn't offer to add type arguments when the kind of literal is ambiguous, but in the case of {} without a context type, the spec resolves the ambiguity by declaring it to be a map, so I think it's fine to add type arguments to var map = {};. It just should have produced var map = <dynamic, dynamic>{}; instead.

[FMorschel]

Here is the CL https://dart-review.googlesource.com/c/sdk/+/412540.