Fix duplicated isolates for the same compilation id (#2308)

Author: ntkme

lib/src/embedded/isolate_dispatcher.dart

@@ -33,7 +33,7 @@ class IsolateDispatcher {
   final _inactiveIsolates = <ReusableIsolate>{};
 
   /// A map from active compilationIds to isolates running those compilations.
-  final _activeIsolates = <int, ReusableIsolate>{};
+  final _activeIsolates = <int, Future<ReusableIsolate>>{};
 
   /// A pool controlling how many isolates (and thus concurrent compilations)
   /// may be live at once.
@@ -54,8 +54,8 @@ class IsolateDispatcher {
         (compilationId, messageBuffer) = parsePacket(packet);
 
         if (compilationId != 0) {
-          var isolate = _activeIsolates[compilationId] ??
-              await _getIsolate(compilationId);
+          var isolate = await _activeIsolates.putIfAbsent(
+              compilationId, () => _getIsolate(compilationId!));
           try {
             isolate.send(packet);
             return;
@@ -108,7 +108,6 @@ class IsolateDispatcher {
       _allIsolates.add(isolate);
     }
 
-    _activeIsolates[compilationId] = isolate;
     isolate.checkOut().listen(_channel.sink.add,
         onError: (Object error, StackTrace stackTrace) {
       if (error is ProtocolError) {