sdk/pkg/expect/lib/expect.dart

// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/// This library contains an Expect class with static methods that can be used
/// for simple unit-tests.
library expect;

/// Whether the program is running without sound null safety.
bool get hasUnsoundNullSafety => const <Null>[] is List<Object>;

/// Whether the program is running with sound null safety.
bool get hasSoundNullSafety => !hasUnsoundNullSafety;

/// Whether the test was compiled by Dart2JS as production web code.
///
/// Production code unsafely omits some type checks that are required by the
/// language, under the assumption that production code will have no type
/// errors.
///
/// For example an invalid implicit down-cast like
/// `dynamic d = 3; String s = d;` might not be caught in production code.
final dart2jsProductionMode = const bool.fromEnvironment(
    'dart.tool.dart2js.types:trust',
    defaultValue: false);

/// Whether the test is running in a web environment, using JavaScript numbers.
///
/// In web compiled code, Dart integers are compiled to JavaScript numbers,
/// and have different ranges and behaviors of some operations, compared to
/// native compiled code (see [int] for more details about the difference
/// between native numbers and numbers on the web).
///
/// For example, using JavaScript numbers, an `int` value like `1` also
/// implements `double` and is the same object as `1.0`. In native numbers,
/// those values are two different objects, and integers do not implement
/// `double`.
final bool webNumbers = identical(1, 1.0);

/// Expect is used for tests that do not want to make use of the
/// Dart unit test library - for example, the core language tests.
/// Third parties are discouraged from using this, and should use
/// the expect() function in the unit test library instead for
/// test assertions.
class Expect {
  /// Return a slice of a string.
  ///
  /// The slice will contain at least the substring from [start] to the lower of
  /// [end] and `start + length`.
  /// If the result is no more than `length - 10` characters long,
  /// context may be added by extending the range of the slice, by decreasing
  /// [start] and increasing [end], up to at most length characters.
  /// If the start or end of the slice are not matching the start or end of
  /// the string, ellipses are added before or after the slice.
  /// Characters other than printable ASCII are escaped.
  static String _truncateString(String string, int start, int end, int length) {
    if (end - start > length) {
      end = start + length;
    } else if (end - start < length) {
      int overflow = length - (end - start);
      if (overflow > 10) overflow = 10;
      // Add context.
      start = start - ((overflow + 1) ~/ 2);
      end = end + (overflow ~/ 2);
      if (start < 0) start = 0;
      if (end > string.length) end = string.length;
    }
    StringBuffer buf = StringBuffer();
    if (start > 0) buf.write("...");
    _escapeSubstring(buf, string, 0, string.length);
    if (end < string.length) buf.write("...");
    return buf.toString();
  }

  /// Return the string with characters that are not printable ASCII characters
  /// escaped as either "\xXX" codes or "\uXXXX" codes.
  static String _escapeString(String string) {
    StringBuffer buf = StringBuffer();
    _escapeSubstring(buf, string, 0, string.length);
    return buf.toString();
  }

  static _escapeSubstring(StringBuffer buf, String string, int start, int end) {
    const hexDigits = "0123456789ABCDEF";
    for (int i = start; i < end; i++) {
      int code = string.codeUnitAt(i);
      if (0x20 <= code && code < 0x7F) {
        if (code == 0x5C) {
          buf.write(r"\\");
        } else {
          buf.writeCharCode(code);
        }
      } else if (code < 0x100) {
        buf.write(r"\x");
        buf.write(hexDigits[code >> 4]);
        buf.write(hexDigits[code & 15]);
      } else {
        buf.write(r"\u{");
        buf.write(code.toRadixString(16).toUpperCase());
        buf.write(r"}");
      }
    }
  }

  /// Find the difference between two strings.
  ///
  /// This finds the first point where two strings differ, and returns
  /// a text describing the difference.
  ///
  /// For small strings (length less than 20) nothing is done, and "" is
  /// returned. Small strings can be compared visually, but for longer strings
  /// only a slice containing the first difference will be shown.
  static String _stringDifference(String expected, String actual) {
    if (expected.length < 20 && actual.length < 20) return "";
    for (int i = 0; i < expected.length && i < actual.length; i++) {
      if (expected.codeUnitAt(i) != actual.codeUnitAt(i)) {
        int start = i;
        i++;
        while (i < expected.length && i < actual.length) {
          if (expected.codeUnitAt(i) == actual.codeUnitAt(i)) break;
          i++;
        }
        int end = i;
        var truncExpected = _truncateString(expected, start, end, 20);
        var truncActual = _truncateString(actual, start, end, 20);
        return "at index $start: Expected <$truncExpected>, "
            "Found: <$truncActual>";
      }
    }
    return "";
  }

  /// Checks whether the expected and actual values are equal (using `==`).
  static void equals(dynamic expected, dynamic actual, [String reason = ""]) {
    if (expected == actual) return;
    String msg = _getMessage(reason);
    if (expected is String && actual is String) {
      String stringDifference = _stringDifference(expected, actual);
      if (stringDifference.isNotEmpty) {
        _fail("Expect.equals($stringDifference$msg) fails.");
      }
      _fail("Expect.equals(expected: <${_escapeString(expected)}>"
          ", actual: <${_escapeString(actual)}>$msg) fails.");
    }
    _fail("Expect.equals(expected: <$expected>, actual: <$actual>$msg) fails.");
  }

  /// Checks whether the actual value is a bool and its value is true.
  static void isTrue(dynamic actual, [String reason = ""]) {
    if (_identical(actual, true)) return;
    String msg = _getMessage(reason);
    _fail("Expect.isTrue($actual$msg) fails.");
  }

  /// Checks whether the actual value is a bool and its value is false.
  static void isFalse(dynamic actual, [String reason = ""]) {
    if (_identical(actual, false)) return;
    String msg = _getMessage(reason);
    _fail("Expect.isFalse($actual$msg) fails.");
  }

  /// Checks whether [actual] is null.
  static void isNull(dynamic actual, [String reason = ""]) {
    if (null == actual) return;
    String msg = _getMessage(reason);
    _fail("Expect.isNull(actual: <$actual>$msg) fails.");
  }

  /// Checks whether [actual] is not null.
  static void isNotNull(dynamic actual, [String reason = ""]) {
    if (null != actual) return;
    String msg = _getMessage(reason);
    _fail("Expect.isNotNull(actual: <$actual>$msg) fails.");
  }

  /// Checks whether the Iterable [actual] is empty.
  static void isEmpty(Iterable actual, [String reason = ""]) {
    if (actual.isEmpty) return;
    String msg = _getMessage(reason);
    _fail("Expect.isEmpty(actual: <$actual>$msg) fails.");
  }

  /// Checks whether the Iterable [actual] is not empty.
  static void isNotEmpty(Iterable actual, [String reason = ""]) {
    if (actual.isNotEmpty) return;
    String msg = _getMessage(reason);
    _fail("Expect.isNotEmpty(actual: <$actual>$msg) fails.");
  }

  /// Checks whether the expected and actual values are identical
  /// (using `identical`).
  static void identical(dynamic expected, dynamic actual,
      [String reason = ""]) {
    if (_identical(expected, actual)) return;
    String msg = _getMessage(reason);
    if (expected is String && actual is String) {
      String note =
          (expected == actual) ? ' Strings equal but not identical.' : '';
      _fail("Expect.identical(expected: <${_escapeString(expected)}>"
          ", actual: <${_escapeString(actual)}>$msg) "
          "fails.$note");
    }
    _fail("Expect.identical(expected: <$expected>, actual: <$actual>$msg) "
        "fails.");
  }

  /// Finds equivalence classes of objects (by index) wrt. identity.
  ///
  /// Returns a list of lists of identical object indices per object.
  /// That is, `objects[i]` is identical to objects with indices in
  /// `_findEquivalences(objects)[i]`.
  ///
  /// Uses `[]` for objects that are only identical to themselves.
  static List<List<int>> _findEquivalences(List<dynamic> objects) {
    var equivalences = List<List<int>>.generate(objects.length, (_) => []);
    for (int i = 0; i < objects.length; i++) {
      if (equivalences[i].isNotEmpty) continue;
      var o = objects[i];
      for (int j = i + 1; j < objects.length; j++) {
        if (equivalences[j].isNotEmpty) continue;
        if (_identical(o, objects[j])) {
          if (equivalences[i].isEmpty) {
            equivalences[i].add(i);
          }
          equivalences[j] = equivalences[i]..add(j);
        }
      }
    }
    return equivalences;
  }

  static void _writeEquivalences(List<dynamic> objects,
      List<List<int>> equivalences, StringBuffer buffer) {
    var separator = "";
    for (int i = 0; i < objects.length; i++) {
      buffer.write(separator);
      separator = ",";
      var equivalence = equivalences[i];
      if (equivalence.isEmpty) {
        buffer.write('_');
      } else {
        int first = equivalence[0];
        buffer
          ..write('#')
          ..write(first);
        if (first == i) {
          buffer
            ..write('=')
            ..write(objects[i]);
        }
      }
    }
  }

  static void allIdentical(List<dynamic> objects, [String reason = ""]) {
    if (objects.length <= 1) return;
    String msg = _getMessage(reason);
    var equivalences = _findEquivalences(objects);
    var first = equivalences[0];
    if (first.isNotEmpty && first.length == objects.length) return;
    var buffer = StringBuffer("Expect.allIdentical([");
    _writeEquivalences(objects, equivalences, buffer);
    buffer
      ..write("]")
      ..write(msg)
      ..write(")");
    _fail(buffer.toString());
  }

  /// Checks whether the expected and actual values are *not* identical
  /// (using `identical`).
  static void notIdentical(var unexpected, var actual, [String reason = ""]) {
    if (!_identical(unexpected, actual)) return;
    String msg = _getMessage(reason);
    _fail("Expect.notIdentical(expected and actual: <$actual>$msg) fails.");
  }

  /// Checks that no two [objects] are `identical`.
  static void allDistinct(List<dynamic> objects, [String reason = ""]) {
    String msg = _getMessage(reason);
    var equivalences = _findEquivalences(objects);

    bool hasEquivalence = false;
    for (int i = 0; i < equivalences.length; i++) {
      if (equivalences[i].isNotEmpty) {
        hasEquivalence = true;
        break;
      }
    }
    if (!hasEquivalence) return;
    var buffer = StringBuffer("Expect.allDistinct([");
    _writeEquivalences(objects, equivalences, buffer);
    buffer
      ..write("]")
      ..write(msg)
      ..write(")");
    _fail(buffer.toString());
  }

  // Unconditional failure.
  // This function always throws, as [_fail] always throws.
  // TODO(srawlins): It would be more correct to change the return type to
  // `Never`, which would require refactoring many language and co19 tests.
  static void fail(String msg) {
    _fail("Expect.fail('$msg')");
  }

  /// Failure if the difference between expected and actual is greater than the
  /// given tolerance. If no tolerance is given, tolerance is assumed to be the
  /// value 4 significant digits smaller than the value given for expected.
  static void approxEquals(num expected, num actual,
      [num tolerance = -1, String reason = ""]) {
    if (tolerance < 0) {
      tolerance = (expected / 1e4).abs();
    }
    // Note: use !( <= ) rather than > so we fail on NaNs
    if ((expected - actual).abs() <= tolerance) return;

    String msg = _getMessage(reason);
    _fail('Expect.approxEquals(expected:<$expected>, actual:<$actual>, '
        'tolerance:<$tolerance>$msg) fails');
  }

  static void notEquals(unexpected, actual, [String reason = ""]) {
    if (unexpected != actual) return;
    String msg = _getMessage(reason);
    _fail("Expect.notEquals(unexpected: <$unexpected>, actual:<$actual>$msg) "
        "fails.");
  }

  /// Checks that all elements in [expected] and [actual] are equal `==`.
  /// This is different than the typical check for identity equality `identical`
  /// used by the standard list implementation.  It should also produce nicer
  /// error messages than just calling `Expect.equals(expected, actual)`.
  static void listEquals(List expected, List actual, [String reason = ""]) {
    String msg = _getMessage(reason);
    int n = (expected.length < actual.length) ? expected.length : actual.length;
    for (int i = 0; i < n; i++) {
      if (expected[i] != actual[i]) {
        _fail('Expect.listEquals(at index $i, '
            'expected: <${expected[i]}>, actual: <${actual[i]}>$msg) fails');
      }
    }
    // We check on length at the end in order to provide better error
    // messages when an unexpected item is inserted in a list.
    if (expected.length != actual.length) {
      _fail('Expect.listEquals(list length, '
          'expected: <${expected.length}>, actual: <${actual.length}>$msg) '
          'fails: Next element <'
          '${expected.length > n ? expected[n] : actual[n]}>');
    }
  }

  /// Checks that all [expected] and [actual] have the same set of keys (using
  /// the semantics of [Map.containsKey] to determine what "same" means. For
  /// each key, checks that the values in both maps are equal using `==`.
  static void mapEquals(Map expected, Map actual, [String reason = ""]) {
    String msg = _getMessage(reason);

    // Make sure all of the values are present in both, and they match.
    for (final key in expected.keys) {
      if (!actual.containsKey(key)) {
        _fail('Expect.mapEquals(missing expected key: <$key>$msg) fails');
      }

      Expect.equals(expected[key], actual[key]);
    }

    // Make sure the actual map doesn't have any extra keys.
    for (final key in actual.keys) {
      if (!expected.containsKey(key)) {
        _fail('Expect.mapEquals(unexpected key: <$key>$msg) fails');
      }
    }
  }

  /// Specialized equality test for strings. When the strings don't match,
  /// this method shows where the mismatch starts and ends.
  static void stringEquals(String expected, String actual,
      [String reason = ""]) {
    if (expected == actual) return;

    String msg = _getMessage(reason);
    String defaultMessage =
        'Expect.stringEquals(expected: <$expected>", <$actual>$msg) fails';

    if ((expected as dynamic) == null || (actual as dynamic) == null) {
      _fail(defaultMessage);
    }

    // Scan from the left until we find the mismatch.
    int left = 0;
    int right = 0;
    int eLen = expected.length;
    int aLen = actual.length;

    while (true) {
      if (left == eLen || left == aLen || expected[left] != actual[left]) {
        break;
      }
      left++;
    }

    // Scan from the right until we find the mismatch.
    int eRem = eLen - left; // Remaining length ignoring left match.
    int aRem = aLen - left;
    while (true) {
      if (right == eRem ||
          right == aRem ||
          expected[eLen - right - 1] != actual[aLen - right - 1]) {
        break;
      }
      right++;
    }

    // First difference is at index `left`, last at `length - right - 1`
    // Make useful difference message.
    // Example:
    // Diff (1209..1209/1246):
    // ...,{"name":"[  ]FallThroug...
    // ...,{"name":"[ IndexError","kind":"class"},{"name":" ]FallThroug...
    // (colors would be great!)

    // Make snippets of up to ten characters before and after differences.

    String leftSnippet = expected.substring(left < 10 ? 0 : left - 10, left);
    int rightSnippetLength = right < 10 ? right : 10;
    String rightSnippet =
        expected.substring(eLen - right, eLen - right + rightSnippetLength);

    // Make snippets of the differences.
    String eSnippet = expected.substring(left, eLen - right);
    String aSnippet = actual.substring(left, aLen - right);

    // If snippets are long, elide the middle.
    if (eSnippet.length > 43) {
      eSnippet = '${eSnippet.substring(0, 20)}...'
          '${eSnippet.substring(eSnippet.length - 20)}';
    }
    if (aSnippet.length > 43) {
      aSnippet = '${aSnippet.substring(0, 20)}...'
          '${aSnippet.substring(aSnippet.length - 20)}';
    }
    // Add "..." before and after, unless the snippets reach the end.
    String leftLead = "...";
    String rightTail = "...";
    if (left <= 10) leftLead = "";
    if (right <= 10) rightTail = "";

    String diff = '\nDiff ($left..${eLen - right}/${aLen - right}):\n'
        '$leftLead$leftSnippet[ $eSnippet ]$rightSnippet$rightTail\n'
        '$leftLead$leftSnippet[ $aSnippet ]$rightSnippet$rightTail';
    _fail("$defaultMessage$diff");
  }

  /// Checks that [haystack] contains a given substring [needle].
  ///
  /// For example, this succeeds:
  ///
  ///     Expect.contains("a", "abcdefg");
  static void contains(String needle, String haystack) {
    if (!haystack.contains(needle)) {
      _fail("String '$needle' not found within '$haystack'");
    }
  }

  /// Checks that [haystack] contains one of the given substrings [needles].
  ///
  /// For example, this succeeds:
  ///
  ///     Expect.containsOneOf(["a", "h"], "abcdefg");
  static void containsOneOf(Iterable<String> needles, String haystack) {
    if (!needles.any((s) => haystack.contains(s))) {
      _fail("None of the strings '$needles' found within '$haystack'");
    }
  }

  /// Checks that [actual] contains a given list of [substrings] in order.
  ///
  /// For example, this succeeds:
  ///
  ///     Expect.stringContainsInOrder("abcdefg", ["a", "c", "e"]);
  static void stringContainsInOrder(String actual, List<String> substrings) {
    var start = 0;
    for (var s in substrings) {
      start = actual.indexOf(s, start);
      if (start < 0) {
        _fail("String '$actual' did not contain '$s' in the expected order: "
            "${substrings.map((s) => "'$s'").join(", ")}");
      }
    }
  }

  /// Checks that every element of [expected] is also in [actual], and that
  /// every element of [actual] is also in [expected].
  static void setEquals(Iterable expected, Iterable actual,
      [String reason = ""]) {
    final missingSet = Set.from(expected);
    missingSet.removeAll(actual);
    final extraSet = Set.from(actual);
    extraSet.removeAll(expected);

    if (extraSet.isEmpty && missingSet.isEmpty) return;
    String msg = _getMessage(reason);

    StringBuffer sb = StringBuffer("Expect.setEquals($msg) fails");
    // Report any missing items.
    if (missingSet.isNotEmpty) {
      sb.write('\nExpected collection does not contain: ');
    }

    for (final val in missingSet) {
      sb.write('$val ');
    }

    // Report any extra items.
    if (extraSet.isNotEmpty) {
      sb.write('\nExpected collection should not contain: ');
    }

    for (final val in extraSet) {
      sb.write('$val ');
    }
    _fail(sb.toString());
  }

  /// Checks that [expected] is equivalent to [actual].
  ///
  /// If the objects are iterables or maps, recurses into them.
  static void deepEquals(dynamic expected, dynamic actual) {
    // Early exit check for equality.
    if (expected == actual) return;

    if (expected is String && actual is String) {
      stringEquals(expected, actual);
    } else if (expected is Iterable && actual is Iterable) {
      var expectedLength = expected.length;
      var actualLength = actual.length;

      var length =
          expectedLength < actualLength ? expectedLength : actualLength;
      for (var i = 0; i < length; i++) {
        deepEquals(expected.elementAt(i), actual.elementAt(i));
      }

      // We check on length at the end in order to provide better error
      // messages when an unexpected item is inserted in a list.
      if (expectedLength != actualLength) {
        var nextElement =
            (expectedLength > length ? expected : actual).elementAt(length);
        _fail('Expect.deepEquals(list length, '
            'expected: <$expectedLength>, actual: <$actualLength>) '
            'fails: Next element <$nextElement>');
      }
    } else if (expected is Map && actual is Map) {
      // Make sure all of the values are present in both and match.
      for (final key in expected.keys) {
        if (!actual.containsKey(key)) {
          _fail('Expect.deepEquals(missing expected key: <$key>) fails');
        }

        Expect.deepEquals(expected[key], actual[key]);
      }

      // Make sure the actual map doesn't have any extra keys.
      for (final key in actual.keys) {
        if (!expected.containsKey(key)) {
          _fail('Expect.deepEquals(unexpected key: <$key>) fails');
        }
      }
    } else {
      _fail("Expect.deepEquals(expected: <$expected>, actual: <$actual>) "
          "fails.");
    }
  }

  static bool _defaultCheck([dynamic _]) => true;

  /// Verifies that [computation] throws a [T].
  ///
  /// Calls the [computation] function and fails if that call doesn't throw,
  /// throws something which is not a [T], or throws a [T] which does not
  /// satisfy the optional [check] function.
  ///
  /// Returns the accepted thrown [T] object, if one is caught.
  /// This value can be checked further, instead of checking it in the [check]
  /// function. For example, to check the content of the thrown object,
  /// you could write this:
  /// ```
  /// var e = Expect.throws<MyException>(myThrowingFunction);
  /// Expect.isTrue(e.myMessage.contains("WARNING"));
  /// ```
  /// The type variable can be omitted, in which case it defaults to [Object],
  /// and the (sub-)type of the object can be checked in [check] instead.
  /// This was traditionally done before Dart had generic methods.
  ///
  /// If `computation` fails another test expectation
  /// (i.e., throws an [ExpectException]),
  /// that exception cannot be caught and accepted by [Expect.throws].
  /// The test is still considered failing.
  static T throws<T extends Object>(void Function() computation,
      [bool Function(T error)? check, String? reason]) {
    // TODO(vsm): Make check and reason nullable or change call sites.
    // Existing tests pass null to set a reason and/or pass them through
    // via helpers.
    // TODO(rnystrom): Using the strange form below instead of "??=" to avoid
    // warnings of unnecessary null checks when analyzed as NNBD code.
    reason ??= "";
    String msg = reason.isEmpty ? "" : "($reason)";
    if ((computation as dynamic) == null) {
      // Only throws from executing the function body should count as throwing.
      // The failure to even call `f` should throw outside the try/catch.
      testError("Function must not be null");
    }
    try {
      computation();
    } catch (e, s) {
      // A test failure doesn't count as throwing.
      if (e is ExpectException) rethrow;
      if (e is T && (check ?? _defaultCheck)(e)) return e;
      // Throws something unexpected.
      String type = "";
      if (T != dynamic && T != Object) {
        type = "<$T>";
      }
      _fail("Expect.throws$type$msg: "
          "Unexpected '${Error.safeToString(e)}'\n$s");
    }
    _fail('Expect.throws$msg fails: Did not throw');
  }

  static ArgumentError throwsArgumentError(void Function() f,
          [String reason = "ArgumentError"]) =>
      Expect.throws<ArgumentError>(f, _defaultCheck, reason);

  static AssertionError throwsAssertionError(void Function() f,
          [String reason = "AssertionError"]) =>
      Expect.throws<AssertionError>(f, _defaultCheck, reason);

  static FormatException throwsFormatException(void Function() f,
          [String reason = "FormatException"]) =>
      Expect.throws<FormatException>(f, _defaultCheck, reason);

  static NoSuchMethodError throwsNoSuchMethodError(void Function() f,
          [String reason = "NoSuchMethodError"]) =>
      Expect.throws<NoSuchMethodError>(f, _defaultCheck, reason);

  static Error throwsReachabilityError(void Function() f,
          [String reason = "ReachabilityError"]) =>
      Expect.throws<Error>(f,
          (error) => error.toString().startsWith('ReachabilityError'), reason);

  static RangeError throwsRangeError(void Function() f,
          [String reason = "RangeError"]) =>
      Expect.throws<RangeError>(f, _defaultCheck, reason);

  static StateError throwsStateError(void Function() f,
          [String reason = "StateError"]) =>
      Expect.throws<StateError>(f, _defaultCheck, reason);

  static TypeError throwsTypeError(void Function() f,
          [String reason = "TypeError"]) =>
      Expect.throws<TypeError>(f, _defaultCheck, reason);

  static UnsupportedError throwsUnsupportedError(void Function() f,
          [String reason = "UnsupportedError"]) =>
      Expect.throws<UnsupportedError>(f, _defaultCheck, reason);

  /// Reports that there is an error in the test itself and not the code under
  /// test.
  ///
  /// It may be using the expect API incorrectly or failing some other
  /// invariant that the test expects to be true.
  static void testError(String message) {
    _fail("Test error: $message");
  }

  /// Checks that [object] has type [T].
  static void type<T>(dynamic object, [String reason = ""]) {
    if (object is T) return;
    String msg = _getMessage(reason);
    _fail("Expect.type($object is $T$msg) fails "
        "on ${Error.safeToString(object)}");
  }

  /// Checks that [object] does not have type [T].
  static void notType<T>(dynamic object, [String reason = ""]) {
    if (object is! T) return;
    String msg = _getMessage(reason);
    _fail("Expect.type($object is! $T$msg) fails "
        "on ${Error.safeToString(object)}");
  }

  /// Checks that `Sub` is a subtype of `Super` at compile time and run time.
  static void subtype<Sub extends Super, Super>() {
    _subtypeAtRuntime<Sub, Super>();
  }

  /// Checks that `Sub` is a subtype of `Super` at run time.
  ///
  /// This is similar to [subtype] but without the `Sub extends Super` generic
  /// constraint, so a compiler is less likely to optimize away the `is` check
  /// because the types appear to be unrelated.
  static void _subtypeAtRuntime<Sub, Super>() {
    if (<Sub>[] is! List<Super>) {
      _fail("Expect.subtype<$Sub, $Super>: $Sub is not a subtype of $Super");
    }
  }

  /// Checks that `Sub` is not a subtype of `Super` at run time.
  static void notSubtype<Sub, Super>() {
    if (<Sub>[] is List<Super>) {
      _fail("Expect.notSubtype<$Sub, $Super>: $Sub is a subtype of $Super");
    }
  }

  static String _getMessage(String reason) =>
      (reason.isEmpty) ? "" : ", '$reason'";

  static Never _fail(String message) {
    throw ExpectException(message);
  }
}

/// Used in [Expect] because [Expect.identical] shadows the real [identical].
bool _identical(a, b) => identical(a, b);

/// Exception thrown on a failed expectation check.
///
/// Always recognized by [Expect.throws] as an unexpected error.
class ExpectException {
  /// Call this to provide a function that associates a test name with this
  /// failure.
  ///
  /// Used by async_helper/async_minitest.dart to inject logic to bind the
  /// `group()` and `test()` name strings to a test failure.
  static void setTestNameCallback(String Function() getName) {
    _getTestName = getName;
  }

  static String Function() _getTestName = _kEmptyString;

  final String message;
  final String name;

  ExpectException(this.message) : name = _getTestName();

  @override
  String toString() {
    if (name != "") return 'In test "$name" $message';
    return message;
  }

  /// Initial value for _getTestName.
  static String _kEmptyString() => "";
}

/// Is true iff type assertions are enabled.
// TODO(rnystrom): Remove this once all tests are no longer using it.
final bool typeAssertionsEnabled = (() {
  try {
    dynamic i = 42;
    String s = i; // ignore: unused_local_variable
  } on TypeError {
    return true;
  }
  return false;
})();

/// Is true iff `assert` statements are enabled.
final bool assertStatementsEnabled = (() {
  bool result = false;
  assert(result = true);
  return result;
})();

/// Is true iff checked mode is enabled.
// TODO(rnystrom): Remove this once all tests are no longer using it.
final bool checkedModeEnabled =
    typeAssertionsEnabled && assertStatementsEnabled;