Fix lazy-collections doubleFibonaccis #746

Job	Run time
Test Browser on Linux (chrome, disabled)	1m 7s
Test Browser on Linux (chrome, main)	43s
Test Browser on Linux (firefox, disabled)	1m 31s
Test Browser on macOS (safari, default)	1m 24s
Test Browser on macOS (safari, disabled)	1m 41s
Test Browser on Linux (firefox, main)	1m 3s
Test Browser on Linux (chrome, default)	1m 29s
Test Browser on Linux (firefox, default)	1m 42s
Test Build	30s
Test Shell (jsc, disabled)	1m 56s
Test Browser on macOS (safari, main)	1m 23s
Test Shell (spidermonkey, default)	5m 4s
Test Shell (jsc, default)	3m 11s
Test Shell (spidermonkey, main)	49s
Test Shell (v8, default)	2m 47s
Test Shell (jsc, main)	44s
Test Shell (v8, main)	44s
Test Shell (v8, disabled)	1m 38s
Test Shell (spidermonkey, disabled)	2m 14s
	31m 40s

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