Long number literals are unreadable

[lrhn]

Dart allows integer literals up to 64-bits, which means that the following integer literals are allowed:

100000000000000000000
0x4000000000000000

Such a number literal is unreadable, and hard to write. It's practically impossible to read the number of digits, and you can only write the number by counting in your head.

Dart should improve the syntax of number literals to make such numbers more easily readable and writable.

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Proposed solutions: #2 (digit separators)