Checkpoint for Response and Tracer: most tests passing.
More tests working.
Work on tracing ending.
Transform tests are passing.
Error tests passing.
Branching tests pass.
All tests passing.
Merge branch 'master' into tracing

Conflicts:
	lib/runtime/cache.dart
	lib/runtime/transforms.dart
	test/generated/addendum_test.dart
	test/generated/schema_unknown_fields_client.dart
Starting to fit tracing into Multiplexer.
Const factories.
Merge branch 'tracing' into tracing-2
Multiplexer tracing events.
Proxy trace event.
Fixes, patches, upgrades, etc.
Track async operations.
Request template fixes.
Fix comment.
Merge pull request #110 from alxhub/tracing

Streamy pipeline overhaul: Response objects and request tracing.