One small problem... the ! operator is not currently user-definable. Is there a reason for this ?

Nor are || and &&. For those the reason is simple: They are not really operators, but rather a shorthand for more complex control flow (just as the conditional operator). I'm guessing that "!", being the fourth boolean "operator" was made not-user-configurable for consistency with the other three - boolean operators can just not be overridden.

I would suggest a top-level "not", and member functions "and" and "or".
expect(v, not(isNull.or(isEmpty)))

expect(v, new isInstanceOf<num>.and(isPositive));

This comment was originally written by @seaneagan

@1: makes sense, if || and && were overridable, then the RHS would get evaluated conditionally based on whether the operators were overloaded, which would be unpredictable.

@2: +1, as that reads the exact same way as the operators would, "not" clearly reads betters than "isNot":

not(equals(5))
not(contains(5))
not(startsWith("foo"))
// vs.
isNot(equals(5))
isNot(contains(5))
isNot(startsWith("foo"))

... and "and" and "or" read more fluently than "allOf" and "anyOf":

expect(v, isNull.or(isEmpty))
expect(v, new isInstanceOf<num>.and(isPositive));
// vs.
expect(v, anyOf(isNull, isEmpty))
expect(v, allOf(new isInstanceOf<num>, isPositive));

Added Area-Library, Triaged labels.

cc @gramster.

I don't think this will happen before M1, and it comes with some cost. I assume that the expectation is that the base Matcher class will support additional methods for composition:

Match and(Matcher other);
Match or(Matcher other);

This means we need to add extra non-final properties to the Matchers for these, and we can't have const matchers for common cases like we do now (e.g. isTrue).

Also, one thing I don't like about this is you can do weird compositions like:

A.and(B).or(C).and(D).or(E)

This comment was originally written by @seaneagan

I think someone suggested | and & on the mailing list, looks like it didn't make it to this bug. Along with "not", seems pretty nice:

expect(x, not(isNull | isEmpty));
expect(y, not(isNull) & isPositive);
// vs.
expect(x, isNot(anyOf(isNull, isEmpty)));
expect(y, allOf(isNotNull, isPositive));

Removed Type-Defect, Area-Library labels.
Added Type-Enhancement, Area-UnitTest labels.

Added Pkg-Unittest label.

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

Removed Pkg-Unittest label.
Added Pkg-matcher label.

This issue has been moved to dart-lang/matcher#11.