dictionary test + patch

[Lendvai Attila]

:: > how is ensure similar to breakOn:?
:: 
:: Lee probably should explain that.

just a note on the tokenizer breakOn: and ensure:

i catch Exhaustion because it contains information about which stream is
exhausted, so it must be rethrown from the tokenizer stream. and i also
assumed a contract that exhausted streams should return Nil if the
Exhaustion is continued without a restart.

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