Implement Dart 1.0 LUB algorithm (for interface types) in kernel.

pkg/kernel/lib/heap.dart

+// Copyright (c) 2017, 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.
+
+/// Basic implementation of a heap, with O(log n) insertion and removal.
+abstract class Heap<T> {

[ahe, 2017/05/01 16:50:34]

Should this be in src or somewhere?

[Paul Berry, 2017/05/01 18:12:13]

Good point.  Done.

+  final _items = <T>[];
+
+  bool get isEmpty => _items.isEmpty;
+
+  bool get isNotEmpty => _items.isNotEmpty;
+
+  void add(T item) {
+    int index = _items.length;
+    _items.length += 1;
+    while (index > 0) {
+      T parent = _items[_parentIndex(index)];
+      if (sortsBefore(parent, item)) break;
+      _items[index] = parent;
+      index = _parentIndex(index);
+    }
+    _items[index] = item;
+  }
+
+  T remove() {
+    T removed = _items[0];
+    T orphan = _items.removeLast();
+    if (_items.isNotEmpty) _reInsert(orphan);
+    return removed;
+  }
+
+  /// Client should use a derived class to specify the sort order.
+  bool sortsBefore(T a, T b);
+
+  int _firstChildIndex(int index) {
+    return (index << 1) + 1;
+  }
+
+  int _parentIndex(int index) {
+    return (index - 1) >> 1;
+  }
+
+  void _reInsert(T item) {
+    int index = 0;
+    while (true) {
+      int childIndex = _firstChildIndex(index);
+      if (childIndex >= _items.length) break;
+      T child = _items[childIndex];
+      if (childIndex + 1 < _items.length) {
+        T nextChild = _items[childIndex + 1];
+        if (sortsBefore(nextChild, child)) {
+          child = nextChild;
+          childIndex++;
+        }
+      }
+      if (sortsBefore(item, child)) break;
+      _items[index] = _items[childIndex];
+      index = childIndex;
+    }
+    _items[index] = item;
+  }
+}