Add offset and end to `NodeList`

[FMorschel]

Over at https://dart-review.googlesource.com/c/sdk/+/413520/comment/107d6345_854a2009/, @bwilkerson suggested:

We should maybe consider adding NodeList.offset, NodeList.length, and NodeList.end
You can use combinators.beginToken?.offset and combinators.endToken?.end to get this information.

At that CL, I made an extension for those values. Two things to have in mind here:

• The list can be empty so offset and end should be nullable
• There is already a length getter which means how many items we have inside the list. We should probably think of a better name for this length or not implement it since it can be easily calculated with the other two values. At the CL extension, I've named it nodesLength.

[bwilkerson]

Those two issues might well be why we didn't add these methods before. I'm thinking that we probably don't want to add them after all.

[FMorschel]

If we could say that the list can't ever be empty and use null in those cases (which is about the same, if we have no "list" of nodes in either case), we could assert offset/end. And that should always be enough to calculate the length.

[bwilkerson]

True, but adding offset and end makes the NodeList look like it's an AstNode even though it isn't. It's then confusing when length has a different semantic. I think it's probably better the way it is, with a list-like interface rather than trying to make it more like an AstNode.