Add JSON codecs

Author: robert-ancell

ci/licenses_golden/licenses_flutter

@@ -1177,6 +1177,9 @@ FILE: ../../../flutter/shell/platform/linux/fl_codec.cc
 FILE: ../../../flutter/shell/platform/linux/fl_dart_project.cc
 FILE: ../../../flutter/shell/platform/linux/fl_engine.cc
 FILE: ../../../flutter/shell/platform/linux/fl_engine_private.h
+FILE: ../../../flutter/shell/platform/linux/fl_json_codec.cc
+FILE: ../../../flutter/shell/platform/linux/fl_json_codec_test.cc
+FILE: ../../../flutter/shell/platform/linux/fl_json_method_codec.cc
 FILE: ../../../flutter/shell/platform/linux/fl_method_channel.cc
 FILE: ../../../flutter/shell/platform/linux/fl_method_codec.cc
 FILE: ../../../flutter/shell/platform/linux/fl_renderer.cc
@@ -1192,6 +1195,8 @@ FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_binary_messe
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_codec.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_json_codec.h
+FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_json_method_codec.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_method_channel.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_method_codec.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_standard_codec.h

shell/platform/linux/BUILD.gn

@@ -48,6 +48,8 @@ _public_headers = [
   "public/flutter_linux/fl_dart_project.h",
   "public/flutter_linux/fl_codec.h",
   "public/flutter_linux/fl_engine.h",
+  "public/flutter_linux/fl_json_codec.h",
+  "public/flutter_linux/fl_json_method_codec.h",
   "public/flutter_linux/fl_method_channel.h",
   "public/flutter_linux/fl_method_codec.h",
   "public/flutter_linux/fl_standard_codec.h",
@@ -69,6 +71,8 @@ source_set("flutter_linux") {
     "fl_codec.cc",
     "fl_dart_project.cc",
     "fl_engine.cc",
+    "fl_json_codec.cc",
+    "fl_json_method_codec.cc",
     "fl_method_channel.cc",
     "fl_method_codec.cc",
     "fl_renderer.cc",
@@ -101,6 +105,7 @@ executable("flutter_linux_unittests") {
   testonly = true
 
   sources = [
+    "fl_json_codec_test.cc",
     "fl_standard_codec_test.cc",
   ]