Merge pull request #2031 from tk0miya/stdlib/JSON.load_file #5395

Job	Run time
test (3.0, test)	2m 20s
test (3.1, test)	2m 7s
test (3.2, test)	1m 59s
test (3.3, test)	2m 23s
test (head, test)	2m 7s
test (head, stdlib_test rubocop)	2m 25s
test (3.3, stdlib_test)	2m 3s
test (3.3, test, --enable-frozen-string-literal)	2m 21s
test (3.3, stdlib_test, --enable-frozen-string-literal)	2m 25s
test (3.3, lexer compile confirm_lexer)	3m 14s
test (3.3, rubocop validate test_doc build test_generate_stdlib raap)	2m 46s
test (3.3, typecheck_test)	59s
	27m 9s

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