captureExpression shouldn't be used when compiling arrow functions

[sdegueldre]

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We would expect this code to log 2, but it logs 1: all values from the context that are used by the function are frozen at function definition time.

Originally, this was meant to deal with loop variables, for which we want a new binding at every loop iteration, but it is overzealous and freezes all free variables.

consider the case of slots