Keep incremental compiler state through full restart. (#12535)

* Keep incremental compiler state through full restart.

* Add todo

* Add todo

packages/flutter_tools/lib/src/devfs.dart

@@ -53,6 +53,13 @@ abstract class DevFSContent {
 class DevFSFileContent extends DevFSContent {
   DevFSFileContent(this.file);
 
+  static DevFSFileContent clone(DevFSFileContent fsFileContent) {
+    final DevFSFileContent newFsFileContent = new DevFSFileContent(fsFileContent.file);
+    newFsFileContent._linkTarget = fsFileContent._linkTarget;
+    newFsFileContent._fileStat = fsFileContent._fileStat;
+    return newFsFileContent;
+  }
+
   final FileSystemEntity file;
   FileSystemEntity _linkTarget;
   FileStat _fileStat;
@@ -435,6 +442,15 @@ class DevFS {
       String archivePath;
       if (deviceUri.path.startsWith(assetBuildDirPrefix))
         archivePath = deviceUri.path.substring(assetBuildDirPrefix.length);
+      // When doing full restart in preview-dart-2 mode, copy content so
+      // that isModified does not reset last check timestamp because we
+      // want to report all modified files to incremental compiler next time
+      // user does hot reload.
+      // TODO(aam): Remove this logic once we switch to using incremental
+      // compiler for full application compilation when doing full restart.
+      if (fullRestart && generator != null && content is DevFSFileContent) {
+        content = DevFSFileContent.clone(content);
+      }
       if (content.isModified || (bundleDirty && archivePath != null)) {
         dirtyEntries[deviceUri] = content;
         numBytes += content.size;

packages/flutter_tools/lib/src/run_hot.dart

@@ -335,14 +335,8 @@ class HotRunner extends ResidentRunner {
     }
 
     final Stopwatch restartTimer = new Stopwatch()..start();
-    if (previewDart2) {
-      for (FlutterDevice device in flutterDevices) {
-        // Reset generator so that full kernel file is produced for VM to
-        // restart from.
-        if (device.generator != null)
-          device.generator.reset();
-      }
-    }
+    // TODO(aam): Add generator reset logic once we switch to using incremental
+    // compiler for full application recompilation on restart.
     final bool updatedDevFS = await _updateDevFS(fullRestart: true);
     if (!updatedDevFS)
       return new OperationResult(1, 'DevFS synchronization failed');
@@ -574,6 +568,7 @@ class HotRunner extends ResidentRunner {
       printTrace('Skipping reassemble because all isolates are paused.');
       return new OperationResult(OperationResult.ok.code, reloadMessage);
     }
+    printTrace('Evicting dirty assets');
     await _evictDirtyAssets();
     printTrace('Reassembling application');
     bool reassembleAndScheduleErrors = false;