[ddc] Append hot restart instruction to hot reload rejection reasons.

By adding this message here, any entrypoint that makes use of this for hot reload (e.g. DartPad, Flutter tools/FE server) will all surface this same message. This probably easier than having each one try to detect errors and append its own message.

Change-Id: I5980d530b66da818e29403cc13407095d8b203dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/408380
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

pkg/dev_compiler/lib/src/command/command.dart

@@ -374,7 +374,8 @@ Future<CompilerResult> _compile(List<String> args,
           lastAcceptedComponent, compiledLibraries);
       if (rejectionReasons.isNotEmpty) {
         throw StateError(
-            'Hot reload rejected due to:\n${rejectionReasons.join('\n')}');
+            'Hot reload rejected due to:\n${rejectionReasons.join('\n')}\n'
+            'Use hot restart instead.');
       }
     }
     var sink = File(reloadDeltaKernel).openWrite();

pkg/frontend_server/lib/src/javascript_bundle.dart

@@ -90,7 +90,11 @@ class IncrementalJavaScriptBundler {
       // connected component graph.
       final List<String> errors = _deltaInspector.compareGenerations(
           lastFullComponent, partialComponent);
-      if (errors.isNotEmpty) throw new Exception(errors.join('/n'));
+      if (errors.isNotEmpty) {
+        throw new Exception(errors.join('/n') +
+            '\nHot reload rejected due to unsupported changes. '
+                'Use hot restart instead.');
+      }
     }
     _currentComponent = partialComponent;
     _updateFullComponent(lastFullComponent, partialComponent);