unittest: "in" Matcher

[DartBot]

This issue was originally filed by @seaneagan

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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"));

[sethladd]

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

[gramster]

In r8951
https://chromiumcodereview.appspot.com//10579008

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Added Fixed label.

[kevmoo]

Added Pkg-Unittest label.

[kevmoo]

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

[DartBot]

This issue has been moved to dart-lang/test#137.