Remove N from all expectAsyncN

[DartBot]

<img src="https://avatars.githubusercontent.com/u/264967?v=3" align="left" width="96" height="96"hspace="10"> Issue by amouravski
Originally opened as dart-lang/sdk#9780

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This is generally waiting for issue dart-lang/sdk#2706 , but here's a potential solution.

typedef dynamic async0();
typedef dynamic async1(dynamic a);
typedef dynamic async2(dynamic a, dynamic b);
...

Function expectAsync(Function callback, ...) {
  if (callback is async0) {
    return expectAsync0(callback, ...);
  } else if (callback is async1) {
    return expectAsync1(callback, ...);
  } else if ( ... ) {
    ...
  } else {
    throw "Too many arguments."
  }
}

Probably not worth changing this right now, but maybe in unittest v2.

[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.

[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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Deprecated in dart-lang/sdk@8bcc876

Leaving this issue open to track final removal.

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Set owner to @kevmoo.
Added Started 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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dart-lang/sdk@402da24

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Added Fixed label.