Preserve original kernel file for isolate spawn requests. (#18127)

* Preserve original kernel file for isolate spawn requests.

This change gives incremental kernel files ".incremental.dill" extension thereby preserving original kernel file.

Bug: https://github.com/flutter/flutter/issues/16084
Bug: https://github.com/flutter/flutter/issues/17012
Bug: https://github.com/flutter/flutter/issues/17983

* Factor out naming logic into separate function

* Fix string vs uri

packages/flutter/lib/src/services/asset_bundle.dart

@@ -67,8 +67,8 @@ abstract class AssetBundle {
     final ByteData data = await load(key);
     if (data == null)
       throw new FlutterError('Unable to load asset: $key');
-    if (data.lengthInBytes < 20 * 1024) {
-      // 20KB takes about 6ms to parse on a Pixel 2 XL.
+    if (data.lengthInBytes < 10 * 1024) {
+      // 10KB takes about 3ms to parse on a Pixel 2 XL.
       // See: https://github.com/dart-lang/sdk/issues/31954
       return utf8.decode(data.buffer.asUint8List());
     }

packages/flutter_tools/lib/src/compile.dart

@@ -442,7 +442,7 @@ class ResidentCompiler {
   /// accepted previously so that next call to [recompile] produces complete
   /// kernel file.
   void reset() {
-    _server.stdin.writeln('reset');
+    _server?.stdin?.writeln('reset');
   }
 
   String _mapFilename(String filename) {

packages/flutter_tools/lib/src/devfs.dart

@@ -412,6 +412,7 @@ class DevFS {
     String dillOutputPath,
     bool fullRestart = false,
     String projectRootPath,
+    String pathToReload,
   }) async {
     // Mark all entries as possibly deleted.
     for (DevFSContent content in _entries.values) {
@@ -510,13 +511,12 @@ class DevFS {
               packagesFilePath : _packagesFilePath);
       final String compiledBinary = compilerOutput?.outputFilename;
       if (compiledBinary != null && compiledBinary.isNotEmpty) {
-        final String entryUri = projectRootPath != null ?
-            fs.path.relative(mainPath, from: projectRootPath):
-            mainPath;
-        final Uri kernelUri = fs.path.toUri(entryUri + '.dill');
-        if (!dirtyEntries.containsKey(kernelUri)) {
+        final Uri entryUri = fs.path.toUri(projectRootPath != null ?
+            fs.path.relative(pathToReload, from: projectRootPath):
+            pathToReload);
+        if (!dirtyEntries.containsKey(entryUri)) {
           final DevFSFileContent content = new DevFSFileContent(fs.file(compiledBinary));
-          dirtyEntries[kernelUri] = content;
+          dirtyEntries[entryUri] = content;
           numBytes += content.size;
         }
       }

packages/flutter_tools/lib/src/resident_runner.dart

@@ -381,6 +381,7 @@ class FlutterDevice {
     Set<String> fileFilter,
     bool fullRestart = false,
     String projectRootPath,
+    String pathToReload,
   }) async {
     final Status devFSStatus = logger.startProgress(
       'Syncing files to device ${device.name}...',
@@ -400,6 +401,7 @@ class FlutterDevice {
         fullRestart: fullRestart,
         dillOutputPath: dillOutputPath,
         projectRootPath: projectRootPath,
+        pathToReload: pathToReload
       );
     } on DevFSException {
       devFSStatus.cancel();
@@ -450,6 +452,10 @@ abstract class ResidentRunner {
   String get projectRootPath => _projectRootPath;
   String _mainPath;
   String get mainPath => _mainPath;
+  String getReloadPath({bool fullRestart}) =>
+      debuggingOptions.buildInfo.previewDart2
+          ? mainPath + (fullRestart? '' : '.incremental') + '.dill'
+          : mainPath;
   AssetBundle _assetBundle;
   AssetBundle get assetBundle => _assetBundle;
 

packages/flutter_tools/lib/src/run_hot.dart

@@ -180,7 +180,7 @@ class HotRunner extends ResidentRunner {
       return 3;
     }
     final Stopwatch initialUpdateDevFSsTimer = new Stopwatch()..start();
-    final bool devfsResult = await _updateDevFS();
+    final bool devfsResult = await _updateDevFS(fullRestart: true);
     _addBenchmarkData('hotReloadInitialDevFSSyncMilliseconds',
         initialUpdateDevFSsTimer.elapsed.inMilliseconds);
     if (!devfsResult)
@@ -324,6 +324,7 @@ class HotRunner extends ResidentRunner {
         fileFilter: _dartDependencies,
         fullRestart: fullRestart,
         projectRootPath: projectRootPath,
+        pathToReload: getReloadPath(fullRestart: fullRestart),
       );
       if (!result)
         return false;
@@ -579,7 +580,7 @@ class HotRunner extends ResidentRunner {
     final Stopwatch vmReloadTimer = new Stopwatch()..start();
     try {
       final String entryPath = fs.path.relative(
-        debuggingOptions.buildInfo.previewDart2 ? mainPath + '.dill' : mainPath,
+        getReloadPath(fullRestart: false),
         from: projectRootPath,
       );
       final Completer<Map<String, dynamic>> retrieveFirstReloadReport = new Completer<Map<String, dynamic>>();