Migrate PortMessageChannel to ES6 class (#766)
Migrate PortMessageChannel to ES6 class (#766)

Migrate the //common/js/src/messaging/port-message-channel.js file
from legacy Closure Compiler classes to ES6 classes.

This is a pure refactoring change. It's is part of the effort to
modernize the code base, as recent Closure Compiler versions
stopped supporting the legacy class syntax.
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