Roughly got save files working.

src/components/Emulator.tsx

@@ -27,7 +27,8 @@ type ReceiveEmulatorMessage = (
   { message: 'start' } |
   { message: 'ready' } |
   { message: 'save_state' } |
-  { message: 'load_state' }
+  { message: 'load_state' } |
+  { message: 'save', data:string }
 );
 
 export type EmulatorProps = {
@@ -68,6 +69,14 @@ export const Emulator:React.FC<EmulatorProps> = props => {
           });
           break;
 
+        case 'save':
+          // Download save data
+          const a = document.createElement('a');
+          a.href = `data:application/zip;base64,${msg.data}`;
+          a.download = `${props.gameId}.zip`;
+          a.click();
+          break;
+
         case 'start':
         case 'ready':
         case 'load_state':