stream: fix Writable.toWeb() hang on synchronous drain #2338

Job	Run time
Build slim tarball	1m 31s
x86_64-linux: with shared libraries	26m 29s
aarch64-linux: with shared libraries	22m 14s
x86_64-darwin: with shared libraries	38m 40s
aarch64-darwin: with shared libraries	27m 3s
	1h 55m 57s

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