unittest: "in" Matcher

[DartBot]

<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="96" height="96"hspace="10"> Issue by seaneagan
Originally opened as dart-lang/sdk#3679

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I think an inverse of the "contains" Matcher would be even more useful, since the Collection/Map/String is more likely the pattern you want to match against. Could call it "in":

Matcher in(container);

//e.g.

expect(x, in([1, 2, 3]));
expect(k, in({"x": 1, "y": 2, "z": 3}));
expect(s, in("xyz"));

[DartBot]

<img src="https://avatars.githubusercontent.com/u/5479?v=3" align="left" width="48" height="48"hspace="10"> Comment by sethladd

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cc @gramster.
Removed Type-Defect label.
Added Type-Enhancement, Area-UnitTest, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2762632?v=3" align="left" width="48" height="48"hspace="10"> Comment by gramster

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In dart-lang/sdk@94f7022
https://chromiumcodereview.appspot.com//10579008

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Added Fixed label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Added Pkg-Unittest label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Removed Area-UnitTest label.
Added Area-Pkg label.