Fix imports in `http2_connection.dart` #636

Job	Run time
analyze (3.0.0)	52s
analyze (dev)	1m 32s
test (ubuntu-latest, 3.0.0, vm)	48s
test (ubuntu-latest, 3.0.0, chrome)	3m 30s
test (ubuntu-latest, dev, vm)	49s
test (ubuntu-latest, dev, chrome)	2m 41s
test (macos-latest, 3.0.0, vm)	1m 5s
test (macos-latest, dev, vm)	3m 27s
test (windows-latest, 3.0.0, vm)	1m 20s
test (windows-latest, dev, vm)	1m 9s
	17m 13s

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