about_string.go: fixed contraction, about_control_flow.go: assert("abc"[0] === 'a') causes internal compiler error

about_string.go: fixed contraction
about_control_flow.go: go 1.1.2 darwin/amd64 throws an internal compiler error, with assert("abc"[0] === 'a'). So, I just put the value of "abc"[0] into a variable, and that made the compiler happy.