Avoid scanning huge freelists for large enough free blocks.

tests/standalone/fragmentation_test.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.
+
+// Deliberately fragment the heap and test that GC peformance does not
+// break down.  See https://github.com/dart-lang/sdk/issues/29588
+// Normally runs in about 6-7 seconds on an x64 machine, using about 2.5Gbytes
+// of memory.
+//
+// This test is deliberately CPU-light and so it can make a lot of
+// progress before the concurrent sweepers are done sweeping the heap.
+// In that time there is no freelist and so the issue does not arise.
+// VMOptions=--no-concurrent-sweep
+
+main() {
+  final List<List> arrays = [];
+  // Fill up heap with alternate large-small items.
+  for (int i = 0; i < 500000; i++) {
+    arrays.add(new List(260));
+    arrays.add(new List(1));
+  }
+  // Clear the large items so that the heap is full of 260-word gaps.
+  for (int i = 0; i < arrays.length; i += 2) {
+    arrays[i] = null;
+  }
+  // Allocate a lot of 300-word objects that don't fit in the gaps.
+  for (int i = 0; i < 600000; i++) {
+    arrays.add(new List(300));
+  }
+}