How do I mock or verify a call to print, in Dart unit tests?

I don't think unittest adds anything specific for this, but you can override any top-level function in the scope of your test and capture calls to a log, for example:

var printLog = [];
void print(String s) => printLog.add(s);

main() {
  test('print', () {
    myFuncUnderTest();
    expect(printLog.length, 2);
    expect(printLog[0], contains('hello'));
    // etc...
  });
}

Update: ZoneSpecification allows overriding the print function. By running code-under-test inside a custom zone you can capture calls to print function. For example, the following test redirects all print messages into an in-memory list log:

import 'dart:async';
import 'package:test/test.dart';

var log = [];

main() {
  test('override print', overridePrint(() {
    print('hello world');
    expect(log, ['hello world']);
  }));
}

void Function() overridePrint(void testFn()) => () {
  var spec = new ZoneSpecification(
    print: (_, __, ___, String msg) {
      // Add to log instead of printing to stdout
      log.add(msg);
    }
  );
  return Zone.current.fork(specification: spec).run<void>(testFn);
};