dart2js: elide type information for functions that are not type-tested

[rakudrama]

In type inference we do tracing to see if the Function.apply metadata is unused because k-bounded tracing is exhaustive and does not flow to Function.apply. This lets dart2js omit the metadata for about half of the closures (example - closures passed to List.map).

We should do something similar for the function $signature. All the functions could simply have the type Function, unless they flow to .runtimeType.