Rewrite line number computation in WSL #879

Job	Run time
Test Browser on macOS (safari, main)	53s
Test Shell (v8, disabled)	1m 21s
Test Browser on macOS (safari, default)	1m 36s
Test Browser on Linux (firefox, default)	1m 23s
Test Browser on Linux (chrome, main)	33s
Test Shell (spidermonkey, disabled)	1m 47s
Test Browser on Linux (chrome, default)	1m 20s
Test Shell (spidermonkey, default)	4m 56s
Test Browser on Linux (chrome, disabled)	1m 2s
Test Shell (spidermonkey, main)	41s
Test Shell (jsc, main)	44s
Test Shell (jsc, default)	2m 54s
Test Browser on Linux (firefox, main)	38s
Test Shell (v8, main)	33s
Test Browser on macOS (safari, disabled)	2m 10s
Test Browser on Linux (firefox, disabled)	1m 9s
Test Shell (v8, default)	2m 42s
Test Shell (jsc, disabled)	1m 29s
Test Build	35s
	28m 26s

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