Fix an edge case with nested Chain.capture blocks.

Specifically, when a null stack trace is passed to a completer or stream
controller in nested Chain.capture blocks, substitute the inner block's
chain rather than the outer block's. This is consistent with other
nested capture behavior, and it's useful for migrating to the new test
runner.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1171863002.
3 files changed
tree: eb39f0c4029d4b2f94c3bbb0682ca675a467e1c6
  1. pkgs/