Add FlValue

FlValue is a lightweight object used to contain the value types that Flutter
uses on platform channels.

Author: robert-ancell

ci/licenses_golden/licenses_flutter

@@ -1182,10 +1182,13 @@ FILE: ../../../flutter/shell/platform/linux/fl_renderer.cc
 FILE: ../../../flutter/shell/platform/linux/fl_renderer.h
 FILE: ../../../flutter/shell/platform/linux/fl_renderer_x11.cc
 FILE: ../../../flutter/shell/platform/linux/fl_renderer_x11.h
+FILE: ../../../flutter/shell/platform/linux/fl_value.cc
+FILE: ../../../flutter/shell/platform/linux/fl_value_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_view.cc
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_binary_messenger.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_dart_project.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_engine.h
+FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_value.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_view.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/flutter_linux.h
 FILE: ../../../flutter/shell/platform/windows/angle_surface_manager.cc

shell/platform/linux/BUILD.gn

@@ -47,6 +47,7 @@ _public_headers = [
   "public/flutter_linux/fl_binary_messenger.h",
   "public/flutter_linux/fl_dart_project.h",
   "public/flutter_linux/fl_engine.h",
+  "public/flutter_linux/fl_value.h",
   "public/flutter_linux/fl_view.h",
   "public/flutter_linux/flutter_linux.h",
 ]
@@ -64,6 +65,7 @@ source_set("flutter_linux") {
     "fl_engine.cc",
     "fl_renderer.cc",
     "fl_renderer_x11.cc",
+    "fl_value.cc",
     "fl_view.cc",
   ]
 
@@ -90,6 +92,7 @@ executable("flutter_linux_unittests") {
 
   sources = [
     "fl_dart_project_test.cc",
+    "fl_value_test.cc",
   ]
 
   public_configs = [ "//flutter:config" ]