Send RPC request to switch assets directory on hot reload. (#12872)

* Send RPC request to switch assets directory on hot reload.

This is needed to pick up updated assets that are expected to be picked up on hot reload.

* Assert assets directory is not null.

* Better multiple future wait

* Add type annotation

packages/flutter_tools/lib/src/resident_runner.dart

@@ -141,6 +141,15 @@ class FlutterDevice {
     return reports;
   }
 
+  Future<Null> resetAssetDirectory() async {
+    final Uri deviceAssetsDirectoryUri = devFS.baseUri.resolveUri(
+        fs.path.toUri(getAssetBuildDirectory()));
+    assert(deviceAssetsDirectoryUri != null);
+    await Future.wait(views.map(
+      (FlutterView view) => view.setAssetDirectory(deviceAssetsDirectoryUri)
+    ));
+  }
+
   // Lists program elements changed in the most recent reload that have not
   // since executed.
   Future<List<ProgramElement>> unusedChangesInLastReload() async {

packages/flutter_tools/lib/src/run_hot.dart

@@ -516,6 +516,8 @@ class HotRunner extends ResidentRunner {
       int countExpectedReports = 0;
       for (FlutterDevice device in flutterDevices) {
         // List has one report per Flutter view.
+        await device.resetAssetDirectory();
+
         final List<Future<Map<String, dynamic>>> reports = device.reloadSources(
           entryPath,
           pause: pause

packages/flutter_tools/lib/src/vmservice.dart

@@ -1318,6 +1318,15 @@ class FlutterView extends ServiceObject {
     await subscription.cancel();
   }
 
+  Future<Null> setAssetDirectory(Uri assetsDirectory) async {
+    assert(assetsDirectory != null);
+    await owner.vmService.vm.invokeRpc('_flutter.setAssetBundlePath',
+        params: <String, dynamic>{
+          'viewId': id,
+          'assetDirectory': assetsDirectory.toFilePath(windows: false)
+        });
+  }
+
   bool get hasIsolate => _uiIsolate != null;
 
   Future<Null> flushUIThreadTasks() async {