anchor.diff:          expect(Complex.tryParse('1.2e2*i'), Complex(0, 120));
           expect(Complex.tryParse('1.2e-2*I'), Complex(0, 0.012));
         });
  -      test('parse error', () {
  +      test('tryParse (error)', () {
           expect(Complex.tryParse(''), isNull);
           expect(Complex.tryParse('e1'), isNull);
           expect(Complex.tryParse('1ii'), isNull);
  
positive.code:         expect(complex.abs(), 5.0);
          expect(complex.arg(), math.acos(4 / 5) + math.pi / 2);
        });
        test('polar', () {
          final complex = Complex.fromPolar(math.sqrt(2), 3 * math.pi / 4);
          expect(complex.a.roundToDouble(), -1.0);
          expect(complex.b.roundToDouble(), 1.0);
  
negative.code:         expect(complex.abs(), 1.0);
          expect(complex.arg(), math.pi / 2);
        });
        test('fromReal', () {
          final complex = Complex.fromReal(123);
          expect(complex.a, 123);
          expect(complex.b, 0);
  