platform conditionals about clojuredart HOT 3 OPEN

We can't use conditional reading because conditional reading will only pick one feature (out of web and native) while we must compile both to a single codebase.

Then it's going to be a special form.

(dart/sniff
  dart:io (some native code)
  dart:html (some web code))

could we lift these expressions to distinct files and emit conditional imports automatically?

We should also modify ns to allow to specify :when in a require.

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See #128

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Let's ponder about

(dart/sniff
  dart:io (some native code)
  dart:html (some web code))

My original idea was to have it replaced by some function call and the function being defined in a conditionally imported lib.
However this approach is going to create weird cases when for example you have an await or a recur inside the expressions (because then they would apply to the extracted function).

My new idea is to have a new special in "dart sexp" (our representation of the subset of dart code we produce) too. Then we would just walk the top-level code in search of a dart/platform-case and if found we would produce dart/platform-case-free variants that we would dump in conditionally imported helper libs.

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