Improve time to development by initializing frontend_server concurrently with...

Improve time to development by initializing frontend_server concurrently with platform build (#45236)

packages/flutter_tools/lib/src/compile.dart

@@ -468,7 +468,6 @@ abstract class ResidentCompiler {
     List<String> dartDefines,
   }) = DefaultResidentCompiler;
 
-
   /// If invoked for the first time, it compiles Dart script identified by
   /// [mainPath], [invalidatedFiles] list is ignored.
   /// On successive runs [invalidatedFiles] indicates which files need to be

packages/flutter_tools/lib/src/devfs.dart

@@ -487,6 +487,9 @@ class DevFS {
     if (fullRestart) {
       generator.reset();
     }
+    // On a full restart, or on an initial compile for the attach based workflow,
+    // this will produce a full dill. Subsequent invocations will produce incremental
+    // dill files that depend on the invalidated files.
     printTrace('Compiling dart to kernel with ${invalidatedFiles.length} updated files');
     final CompilerOutput compilerOutput = await generator.recompile(
       mainPath,

packages/flutter_tools/lib/src/run_hot.dart

@@ -18,6 +18,7 @@ import 'base/platform.dart';
 import 'base/terminal.dart';
 import 'base/utils.dart';
 import 'build_info.dart';
+import 'bundle.dart';
 import 'compile.dart';
 import 'convert.dart';
 import 'devfs.dart';
@@ -262,14 +263,31 @@ class HotRunner extends ResidentRunner {
 
     firstBuildTime = DateTime.now();
 
+    final List<Future<bool>> startupTasks = <Future<bool>>[];
     for (FlutterDevice device in flutterDevices) {
-      final int result = await device.runHot(
+      // Here we initialize the frontend_server conccurently with the gradle
+      // build, reducing initialization time. This is safe because the first
+      // invocation of the frontend server produces a full dill file that
+      // the subsequent invocation in devfs will not overwrite.
+      if (device.generator != null) {
+        startupTasks.add(
+          device.generator.recompile(
+            mainPath,
+            <Uri>[],
+            outputPath: dillOutputPath ??
+              getDefaultApplicationKernelPath(trackWidgetCreation: device.trackWidgetCreation),
+            packagesFilePath : packagesFilePath,
+          ).then((CompilerOutput output) => output?.errorCount == 0)
+        );
+      }
+      startupTasks.add(device.runHot(
         hotRunner: this,
         route: route,
-      );
-      if (result != 0) {
-        return result;
-      }
+      ).then((int result) => result == 0));
+    }
+    final List<bool> results = await Future.wait(startupTasks);
+    if (!results.every((bool passed) => passed)) {
+      return 1;
     }
 
     return attach(

packages/flutter_tools/test/integration.shard/hot_reload_test.dart

@@ -108,13 +108,18 @@ void main() {
   });
 
   test('hot reload doesn\'t reassemble if paused', () async {
-    await _flutter.run(withDebugger: true);
+    final Future<void> setup = _flutter.run(withDebugger: true);
+    final Completer<void> sawTick1 = Completer<void>();
     final Completer<void> sawTick2 = Completer<void>();
     final Completer<void> sawTick3 = Completer<void>();
     final Completer<void> sawDebuggerPausedMessage1 = Completer<void>();
     final Completer<void> sawDebuggerPausedMessage2 = Completer<void>();
     final StreamSubscription<String> subscription = _flutter.stdout.listen(
       (String line) {
+        if (line.contains('((((TICK 1))))')) {
+          expect(sawTick1.isCompleted, isFalse);
+          sawTick1.complete();
+        }
         if (line.contains('((((TICK 2))))')) {
           expect(sawTick2.isCompleted, isFalse);
           sawTick2.complete();
@@ -129,6 +134,8 @@ void main() {
         }
       },
     );
+    await setup;
+    await sawTick1.future;
     await _flutter.addBreakpoint(
       _project.buildBreakpointUri,
       _project.buildBreakpointLine,