Emit spans with reasonable ends

[nex3]

Currently (unless the user passes in SourceFiles) the spans emitted by this package don't have useful end locations. Non-identifier spans are always point locations, although typically a source map entry is considered to continue until the next entry rather than being a single point. Identifier spans are even worse: their text fields are used to contain the identifier, which is often (usually) not actual text from the source file, which is what the field is for.

Issue dart-lang/source_span#3 will allow these spans to have the correct end position set without necessarily having access to the text of the span.