sync-fs and async-fs improvements (#217) #733

Job	Run time
Test Browser on macOS (safari, default)	1m 42s
Test Browser on macOS (safari, disabled)	1m 45s
Test Browser on macOS (safari, main)	1m 2s
Test Build	25s
Test Browser on Linux (firefox, main)	58s
Test Browser on Linux (chrome, main)	41s
Test Browser on Linux (chrome, default)	1m 16s
Test Browser on Linux (firefox, default)	1m 41s
Test Browser on Linux (chrome, disabled)	1m 13s
Test Browser on Linux (firefox, disabled)	1m 21s
Test Shell (v8, default)	3m 7s
Test Shell (jsc, main)	58s
Test Shell (jsc, disabled)	2m 3s
Test Shell (spidermonkey, default)	5m 25s
Test Shell (spidermonkey, disabled)	2m 17s
Test Shell (v8, disabled)	1m 44s
Test Shell (jsc, default)	3m 37s
Test Shell (v8, main)	46s
Test Shell (spidermonkey, main)	53s
	32m 54s

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