Provide content tests with 'in'

[DartBot]

This issue was originally filed by jk...@gmail.com

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Basically I think it would be nice to extend the use of in from just for loops to also be used inside statements for checking content

    int a = 44
    print(a in [1,3,44]);
    // true

This would give 'in' more uses and would mean that content checks are often cleaner to read.

Let me know what you think

[floitschG]

Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[floitschG]

In other words: "x in y" would be syntactic sugar for "y.contains(x)".

[floitschG]

Added Duplicate label.
Marked as being merged into #510.