matchers: nicer syntax for Type testing

[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#10407

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What steps will reproduce the problem?
1.
2.
3.

What is the expected output? What do you see instead?

What version of the product are you using? On what operating system?

currently it doesn't read as nicely as other matchers:

expect(x, new isInstanceOf<int>());
expect(f, throwsA(new isInstanceOf<ExpectedError>()));

better would be:

expect(x, isA(int));
expect(f, throwsA(isA(ExpectedError)));

or even:

expect(x, int);
expect(f, throwsA(ExpectedError));

I think this is blocked on issue dart-lang/sdk#10406.

Please provide any additional information below.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2909286?v=3" align="left" width="48" height="48"hspace="10"> Comment by madsager

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cc @sigmundch.
Added Area-Pkg, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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cc @gramster.
Removed Area-Pkg label.
Added Area-unittest label.

[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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Initially we took this approach because we could not rely on 'is' in dart2js if the type was passed as a parameter. Perhaps there is enough reflection support now to make this work; will have to investigate.

In the meantime, a workaround is to define matcher instances for the types you care about (the library already does this for most exception and error types). E.g.

final is_int = new predicate((x) => x is int, "is an int");

[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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Added Duplicate label.
Marked as being merged into dart-lang/sdk#8138.

[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.