[vm, isolates] Allocate the main port before entering the isolate.

Once we enter the isolate, other threads may read the main port as part of attempting to bring all the group members to a reload safepoint.

TEST=tsan
Change-Id: Ie7ccfd8672c9b923c7e071023a418ddbc58660ae
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/398780
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/isolate.cc

@@ -1860,14 +1860,6 @@ Isolate* Isolate::InitIsolate(const char* name_prefix,
 #undef ISOLATE_METRIC_INIT
 #endif  // !defined(PRODUCT)
 
-  // First we ensure we enter the isolate. This will ensure we're participating
-  // in any safepointing requests from this point on. Other threads requesting a
-  // safepoint operation will therefore wait until we've stopped.
-  //
-  // Though the [result] isolate is still in a state where no memory has been
-  // allocated, which means it's safe to GC the isolate group until here.
-  Thread::EnterIsolate(result);
-
   // Setup the isolate message handler.
   result->message_handler_ = new IsolateMessageHandler(result);
 
@@ -1879,6 +1871,14 @@ Isolate* Isolate::InitIsolate(const char* name_prefix,
 #endif
   result->set_origin_id(result->main_port());
 
+  // First we ensure we enter the isolate. This will ensure we're participating
+  // in any safepointing requests from this point on. Other threads requesting a
+  // safepoint operation will therefore wait until we've stopped.
+  //
+  // Though the [result] isolate is still in a state where no memory has been
+  // allocated, which means it's safe to GC the isolate group until here.
+  Thread::EnterIsolate(result);
+
   // Keep capability IDs less than 2^53 so web clients of the service
   // protocol can process it properly.
   //

runtime/vm/port.cc

@@ -59,10 +59,6 @@ Dart_Port PortMap::CreatePort(PortHandler* handler) {
     return ILLEGAL_PORT;
   }
 
-#if defined(DEBUG)
-  handler->CheckAccess();
-#endif
-
   const Dart_Port port = AllocatePort();
   if (auto ports = handler->ports(ml)) {
     ports->Insert(PortHandler::PortSetEntry{port});