Fix bug in Duration.toString.

On the web, some inputs to `Duration` would make -0.0
occur in positions where the code assumed an integer
with a reasonable `toString`.

Adds `+ 0` and using `0 - x` instead of `-x` to
avoid ever ending up with a `-0.0`.

Fixes #51584

CoreLibraryReviewExempt: Bugfix, no API changes.
Change-Id: Idecfaf049f2ae5677792387ce24e95add99596cb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/291280
Commit-Queue: Nate Bosch <nbosch@google.com>
Auto-Submit: Lasse Nielsen <lrn@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>

sdk/lib/_internal/vm_shared/lib/date_patch.dart

@@ -2,7 +2,7 @@
 // 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.
 
-import "dart:_internal" show patch, unsafeCast;
+import "dart:_internal" show checkNotNullable, patch, unsafeCast;
 
 // VM implementation of DateTime.
 @patch
@@ -47,12 +47,11 @@ class DateTime {
   @patch
   DateTime._internal(int year, int month, int day, int hour, int minute,
       int second, int millisecond, int microsecond, bool isUtc)
-      : this.isUtc = isUtc,
+      : this.isUtc = checkNotNullable(isUtc, "isUtc"),
         this._value = _brokenDownDateToValue(year, month, day, hour, minute,
                 second, millisecond, microsecond, isUtc) ??
             -1 {
     if (_value == -1) throw new ArgumentError();
-    if (isUtc == null) throw new ArgumentError();
   }
 
   static int _validateMilliseconds(int millisecondsSinceEpoch) =>

sdk/lib/core/duration.dart

@@ -161,8 +161,9 @@ class Duration implements Comparable<Duration> {
   /// underflow and subtract from the next larger unit.
   ///
   /// If the total number of microseconds cannot be represented
-  /// as an integer value, the number of microseconds might be truncated
-  /// and it might lose precision.
+  /// as an integer value, the number of microseconds might overflow
+  /// and be truncated to a smaller number of bits,
+  /// or it might lose precision.
   ///
   /// All arguments are 0 by default.
   /// ```dart
@@ -184,9 +185,10 @@ class Duration implements Comparable<Duration> {
             microsecondsPerHour * hours +
             microsecondsPerDay * days);
 
-  // Fast path internal direct constructor to avoids the optional arguments and
-  // [_microseconds] recomputation.
-  const Duration._microseconds(this._duration);
+  // Fast path internal direct constructor to avoids the optional arguments
+  // and [_microseconds] recomputation.
+  // The `+ 0` prevents -0.0 on the web, if the incoming duration happens to be -0.0.
+  const Duration._microseconds(int duration) : _duration = duration + 0;
 
   /// Adds this Duration and [other] and
   /// returns the sum as a new Duration object.
@@ -212,7 +214,7 @@ class Duration implements Comparable<Duration> {
   /// Divides this Duration by the given [quotient] and returns the truncated
   /// result as a new Duration object.
   ///
-  /// Throws an [IntegerDivisionByZeroException] if [quotient] is `0`.
+  /// The [quotient] must not be `0`.
   Duration operator ~/(int quotient) {
     // By doing the check here instead of relying on "~/" below we get the
     // exception even with dart2js.
@@ -331,12 +333,19 @@ class Duration implements Comparable<Duration> {
   /// ```
   String toString() {
     var microseconds = inMicroseconds;
-    var sign = (microseconds < 0) ? "-" : "";
+    var sign = "";
+    var negative = microseconds < 0;
 
     var hours = microseconds ~/ microsecondsPerHour;
     microseconds = microseconds.remainder(microsecondsPerHour);
 
-    if (microseconds < 0) microseconds = -microseconds;
+    // Correcting for being negative after first division, instead of before,
+    // to avoid negating min-int, -(2^31-1), of a native int64.
+    if (negative) {
+      hours = 0 - hours; // Not using `-hours` to avoid creating -0.0 on web.
+      microseconds = 0 - microseconds;
+      sign = "-";
+    }
 
     var minutes = microseconds ~/ microsecondsPerMinute;
     microseconds = microseconds.remainder(microsecondsPerMinute);
@@ -348,10 +357,17 @@ class Duration implements Comparable<Duration> {
 
     var secondsPadding = seconds < 10 ? "0" : "";
 
-    var paddedMicroseconds = microseconds.toString().padLeft(6, "0");
-    return "$sign${hours.abs()}:"
+    var microsecondsText = microseconds.toString();
+
+    // Padding up to six digits for microseconds.
+    const zeroPadding = ["00000", "0000", "000", "00", "0", ""];
+    assert(microsecondsText.length >= 1 && microsecondsText.length <= 6);
+    var microsecondsPadding = zeroPadding[microsecondsText.length - 1];
+
+    return "$sign$hours:"
         "$minutesPadding$minutes:"
-        "$secondsPadding$seconds.$paddedMicroseconds";
+        "$secondsPadding$seconds."
+        "$microsecondsPadding$microsecondsText";
   }
 
   /// Whether this [Duration] is negative.

tests/corelib/duration_test.dart

@@ -315,4 +315,19 @@ main() {
   Expect.isFalse(d.toString().startsWith("--"));
   d = const Duration(minutes: -0); // is -0.0 on web.
   Expect.equals("0:00:00.000000", d.toString());
+
+  // Regression test for https://github.com/dart-lang/sdk/issues/51584
+  // (Didn't fix it all in 48841.)
+  d = const Duration(hours: -1);
+  Expect.equals("-1:00:00.000000", d.toString());
+
+  // Adding -0.0's gives -0.0 again.
+  d = const Duration(
+      days: -0,
+      hours: -0,
+      minutes: -0,
+      seconds: -0,
+      milliseconds: -0,
+      microseconds: -0);
+  Expect.equals("0:00:00.000000", d.toString());
 }

tests/corelib_2/duration_test.dart

@@ -317,4 +317,19 @@ main() {
   Expect.isFalse(d.toString().startsWith("--"));
   d = const Duration(minutes: -0); // is -0.0 on web.
   Expect.equals("0:00:00.000000", d.toString());
+
+  // Regression test for https://github.com/dart-lang/sdk/issues/51584
+  // (Didn't fix it all in 48841.)
+  d = const Duration(hours: -1);
+  Expect.equals("-1:00:00.000000", d.toString());
+
+  // Adding -0.0's gives -0.0 again.
+  d = const Duration(
+      days: -0,
+      hours: -0,
+      minutes: -0,
+      seconds: -0,
+      milliseconds: -0,
+      microseconds: -0);
+  Expect.equals("0:00:00.000000", d.toString());
 }