show value in dart vm dynamic type check

[DartBot]

This issue was originally filed by mattsh@google.com

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When I have a failing type check on the dart vm (for example, here I am attempting to assign an integer to a double:

main() {
  double a = 23;
}

it prints:

Failed type check: type 'Smi' is not assignable to type 'double' of 'a'.

This is useful. But it would be good to include the actual value as well. So something like this would be good I think:

Failed type check: type 'Smi' (23) is not assignable to type 'double' of 'a'.

I'm not sure yet of the best formatting exactly, but for now if we could dump the actual value in there somewhere, that would help.

Actually, I notice that class TypeError currently has these fields:
  final String srcType;
  final String dstType;
  final String dstName;

I think we should have one more field to hold the offending value.

[iposva-google]

How much are you willing to print? What if the value you are assigning is an array with 1000 elements? I do not think it is feasible to hold on to the value as this can cause an otherwise dead object to be preserved. We could choose to print the value for well-known types for which we know the implementation of toString.

[iposva-google]

None of the follow up questions were addressed. Closing.

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Added AsDesigned label.