Support "pub downgrade".

sdk/lib/_internal/pub/lib/src/utils.dart

Index: sdk/lib/_internal/pub/lib/src/utils.dart
diff --git a/sdk/lib/_internal/pub/lib/src/utils.dart b/sdk/lib/_internal/pub/lib/src/utils.dart
index 68c0c74188ba3c9add56553c9b6146d2dcf1dad0..fb2fe64c3c44cf2fd64a3d9420b11c4382f14216 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -368,6 +368,15 @@ maxAll(Iterable iter, [int compare(element1, element2)]) {
       compare(element, max) > 0 ? element : max);
 }
 
+/// Returns the minimum value in [iter] by [compare].
+///
+/// [compare] defaults to [Comparable.compare].
+minAll(Iterable iter, [int compare(element1, element2)]) {
+  if (compare == null) compare = Comparable.compare;
+  return iter.reduce((max, element) =>
+      compare(element, max) < 0 ? element : max);
+}
+
 /// Replace each instance of [matcher] in [source] with the return value of
 /// [fn].
 String replace(String source, Pattern matcher, String fn(Match)) {