Use IKG for restarts, use IKG with ProtectedFileByteStore. (#12953)

* Use IKG for restarts, use IKG with ProtectedFileByteStore.

* Fix comment, add end of file newline.

* Remove unused import
diff --git a/packages/flutter_tools/lib/src/build_info.dart b/packages/flutter_tools/lib/src/build_info.dart
index 672c43b..3454552 100644
--- a/packages/flutter_tools/lib/src/build_info.dart
+++ b/packages/flutter_tools/lib/src/build_info.dart
@@ -205,3 +205,9 @@
 String getIosBuildDirectory() {
   return fs.path.join(getBuildDirectory(), 'ios');
 }
+
+/// Returns directory used by incremental compiler (IKG - incremental kernel
+/// generator) to store cached intermediate state.
+String getIncrementalCompilerByteStoreDirectory() {
+  return fs.path.join(getBuildDirectory(), 'ikg_byte_store');
+}