Work around race condition in MakeArray for ASSERTs and document assumptions.

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
===================================================================
--- runtime/vm/raw_object.cc	(revision 43879)
+++ runtime/vm/raw_object.cc	(working copy)
@@ -216,6 +216,18 @@
 #if defined(DEBUG)
   uword tags = ptr()->tags_;
   intptr_t tags_size = SizeTag::decode(tags);
+  if ((class_id == kArrayCid) && (instance_size > tags_size && tags_size > 0)) {
+    // TODO(22501): Array::MakeArray could be in the process of shrinking
+    // the array (see comment therein), having already updated the tags but not
+    // yet set the new length. Wait a millisecond and try again.
+    int retries_remaining = 1000;  // ... but not forever.
+    do {
+      OS::Sleep(1);
+      const RawArray* raw_array = reinterpret_cast<const RawArray*>(this);
+      intptr_t array_length = Smi::Value(raw_array->ptr()->length_);
+      instance_size = Array::InstanceSize(array_length);
+    } while ((instance_size > tags_size) && (--retries_remaining > 0));
+  }
   if ((instance_size != tags_size) && (tags_size != 0)) {
     FATAL3("Size mismatch: %" Pd " from class vs %" Pd " from tags %" Px "\n",
            instance_size, tags_size, tags);