Update isolates.md - TODOs are reversed (dart-lang#5500)

Fixes confusing (because reversed) Todo comments in the mini exercise.

Author: dkbast

examples/concurrency/lib/basic_ports_example/start.dart

@@ -8,11 +8,11 @@ class Worker {
   }
 
   void _handleResponsesFromIsolate(dynamic message) {
-    // TODO: Define code that should be executed on the worker isolate.
+    // TODO: Handle messages sent back from the worker isolate.
   }
 
   static void _startRemoteIsolate(SendPort port) {
-    // TODO: Handle messages sent back from the worker isolate.
+    // TODO: Define code that should be executed on the worker isolate.
   }
 
   Future<void> parseJson(String message) async {

src/language/isolates.md

@@ -310,11 +310,11 @@ class Worker {
   }
 
   void _handleResponsesFromIsolate(dynamic message) {
-    // TODO: Define code that should be executed on the worker isolate.
+    // TODO: Handle messages sent back from the worker isolate.
   }
 
   static void _startRemoteIsolate(SendPort port) {
-    // TODO: Handle messages sent back from the worker isolate.
+    // TODO: Define code that should be executed on the worker isolate.
   }
 
   Future<void> parseJson(String message) async {