dart2js returns true for infinity is int

[DartBot]

This issue was originally filed by ngeoffray@google.com

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See test language/infinity_test.dart. dart2js uses Math.floor(x) == x for making an int check, and infinity returns true to that operation.

[kasperl]

Removed this from the Later milestone.

[kasperl]

Added this to the M2 milestone.

[karlklose]

This is also a problem for -0.0.

[peter-ahe-google]

Removed this from the M2 milestone.
Added this to the Later milestone.

[kasperl]

Added TriageForM5 label.

[kasperl]

Removed TriageForM5 label.

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[kasperl]

Removed Oldschool-Milestone-Later label.

[rakudrama]

We cannot easily fix that Infinity and -0.0 report as 'is int'.
It is really the same issue as #1533

In the VM, int 0 and double 0.0 give different results for negation (0 and -0.0).
This could be achieved by using the JavaScript expression -x for double and 0-x for int.
However, since 0 and 0.0 have the same representation in JavaScript, in a dynamic or num context we can't tell if the value is supposed to be double or int. For consistency we must choose one of the above. If we try to be inconsistent then the inconsistency will surface in unpredictable ways.
We chose that -0.0 is an int (and this value works in JavaScript indexing, bitops etc) rather than 'breaking' negation for doubles.

There is a similar problem with Infinity. What is the type of a+b when a and b are int?
We have a choice of saying that + is not closed on ints, meaning either (1) we model int + int as returning double (leading to many more type checks on operations like indexing), or (2) we check the result and throw on 'overflow' (but when do we check if the static type is num?), or (3) accepting Infinity as an int. Accepting Infinity is the most practical choice for generating fast idiomatic JavaScript.