Pull vm service uri from extension iFrame query parameters (#6335)

packages/devtools_extensions/lib/src/template/extension_manager.dart

@@ -30,23 +30,28 @@ class ExtensionManager {
 
   // ignore: unused_element, false positive due to part files
   void _init({required bool connectToVmService}) {
+    html.window.addEventListener('message', _handleMessage);
+
     // TODO(kenz): handle the ide theme that may be part of the query params.
     final queryParams = loadQueryParams();
     final themeValue = queryParams[ExtensionEventParameters.theme];
     darkThemeEnabled.value = themeValue == null ||
         themeValue == ExtensionEventParameters.themeValueDark;
 
-    html.window.addEventListener('message', _handleMessage);
-    // TODO(kenz) instead of connecting to the VM service through an event, load
-    // the vm service URI through a query parameter like we already do in
-    // DevTools app.
+    final vmServiceUri = queryParams['uri'];
     if (connectToVmService) {
-      // Request the vm service uri for the connected app. DevTools will
-      // respond with a [DevToolsPluginEventType.connectedVmService] event with
-      // containing the currently connected app's vm service URI.
-      postMessageToDevTools(
-        DevToolsExtensionEvent(DevToolsExtensionEventType.vmServiceConnection),
-      );
+      if (vmServiceUri == null) {
+        // Request the vm service uri for the connected app. DevTools will
+        // respond with a [DevToolsPluginEventType.connectedVmService] event
+        // containing the currently connected app's vm service URI.
+        postMessageToDevTools(
+          DevToolsExtensionEvent(
+            DevToolsExtensionEventType.vmServiceConnection,
+          ),
+        );
+      } else {
+        unawaited(_connectToVmService(vmServiceUri));
+      }
     }
   }