Support evaluation at location that don't precisely match known dart location

[annagrin]

Current expression evaluator looks up dart locations matching js locations in locations table in dwds, which are not always available for every js location. We need to figure out ways of solving this problem, at least in most useful cases.

See:

Does this mean we can only evaluate expressions at known locations? Is this a large limitation in practice?

Originally posted by @grouma in https://github.com/_render_node/MDE3OlB1bGxSZXF1ZXN0UmV2aWV3MzUyNTY1MjA1/pull_request_reviews/more_threads