Working on support for propagation via OpenCensus propagation modules in Stackdriver Trace (googleapis/cloud-trace-nodejs#950), I ran into a couple of issues:

• Because strictNullChecks is turned off, B3Format does an unsafe non-null cast (source). The result is that consumers of this propagation library will get invalid values whenever the B3 header is not set, and crash if they depended on these values being valid (which is a reasonable assumption for TS users).
• Not a behavioral bug but there are a few superfluous falsiness checks in B3Format such as for getter and setter.
• The B3 propagation module exports a different interface than the Stackdriver propagation module. Stackdriver propagation exports an implementation of Propagation, while B3 propagation exports a class constructor. I believe they should all export the former rather than the latter.

These issues may also exist for other propagation mechanisms, I only looked at B3 in particular.