Adds [Heap]PriorityQueue.of constructor.

[lrhn]

Introduces efficient (linear-number of comparisons) "heapify" algorithm for converting an unsorted list to a heap-sorted list, using it for the of constructor, and after a large addAll operation, when it's presumed faster than just bubbling down all the new elements.

Also rewrites HeapPriorityQueue to use a growable list as backing array, instead of implementing the same thing using the double-when-full algorithm, and still having to deal with nullable cells. The platform growable list implementation is assumed to efficiently avoid some of those null checks.

Closes #732

[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
collection	Breaking	1.19.1	1.20.0-wip	2.0.0 Got "1.20.0-wip" expected >= "2.0.0" (breaking changes)	⚠️

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Coverage ⚠️

File	Coverage
pkgs/collection/lib/src/priority_queue.dart	💔 98 % ⬇️ 0 %

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License Headers ✔️

// Copyright (c) 2025, 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.

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[lrhn]

Ping @natebosch

[devoncarew]

We're seeing a google3 roll failure, I believe from this change:

[.../async/lib/src/priority_stream_iterator.dart:110]
'_StablePriorityQueue.removeAll' ('Iterable<T> Function()') isn't a valid override of 'HeapPriorityQueue.removeAll' ('List<T> Function()'). #invalid_override

  Iterable<T> removeAll() {
              ^^^^^^^^^