Minor fixes/adjustments to the GLFW shell (flutter/engine#8990)

- Makes json_method_codec.cc compatible with the last stable RapidJSON release.
- Allows removing the GTK dependency with a compile flag.
- Fixes a missing break in a switch flagged by some toolchains.

engine/src/flutter/shell/platform/common/cpp/client_wrapper/json_method_codec.cc

@@ -44,7 +44,12 @@ JsonMethodCodec::DecodeMethodCallInternal(const uint8_t* message,
     // Pull the arguments subtree up to the root of json_message. This is
     // destructive to json_message, but the full value is no longer needed, and
     // this avoids a subtree copy.
-    json_message->Swap(arguments_iter->value);
+    // Note: The static_cast is for compatibility with RapidJSON 1.1; master
+    // already allows swapping a Document with a Value directly. Once there is
+    // a new RapidJSON release (at which point clients can be expected to have
+    // that change in the version they depend on) remove the cast.
+    static_cast<rapidjson::Value*>(json_message.get())
+        ->Swap(arguments_iter->value);
     // Swap it into |arguments|. This moves the allocator ownership, so that
     // the data won't be deleted when json_message goes out of scope.
     arguments = std::make_unique<rapidjson::Document>();

engine/src/flutter/shell/platform/glfw/flutter_glfw.cc

@@ -19,7 +19,14 @@
 #include "flutter/shell/platform/glfw/keyboard_hook_handler.h"
 #include "flutter/shell/platform/glfw/text_input_plugin.h"
 
-#ifdef __linux__
+// For compatibility with GTK-based plugins, special message loop setup is
+// required (e.g., for GTK modal windows). This can be disabled for cases where
+// a GTK dependency is undesirable by defining FLUTTER_DISABLE_GTK.
+#if defined(__linux__) && !defined(FLUTTER_DISABLE_GTK)
+#define FLUTTER_USE_GTK 1
+#endif
+
+#ifdef FLUTTER_USE_GTK
 // For plugin-compatible event handling (e.g., modal windows).
 #include <X11/Xlib.h>
 #include <gtk/gtk.h>
@@ -519,7 +526,7 @@ FlutterDesktopWindowControllerRef FlutterDesktopCreateWindow(
     const char* icu_data_path,
     const char** arguments,
     size_t argument_count) {
-#ifdef __linux__
+#ifdef FLUTTER_USE_GTK
   gtk_init(0, nullptr);
 #endif
 
@@ -659,13 +666,13 @@ double FlutterDesktopWindowGetScaleFactor(
 
 void FlutterDesktopRunWindowLoop(FlutterDesktopWindowControllerRef controller) {
   GLFWwindow* window = controller->window.get();
-#ifdef __linux__
+#ifdef FLUTTER_USE_GTK
   // Necessary for GTK thread safety.
   XInitThreads();
 #endif
   while (!glfwWindowShouldClose(window)) {
     glfwPollEvents();
-#ifdef __linux__
+#ifdef FLUTTER_USE_GTK
     if (gtk_events_pending()) {
       gtk_main_iteration();
     }

engine/src/flutter/shell/platform/glfw/text_input_plugin.cc

@@ -86,6 +86,7 @@ void TextInputPlugin::KeyboardHook(GLFWwindow* window,
         break;
       case GLFW_KEY_ENTER:
         EnterPressed(active_model_);
+        break;
       default:
         break;
     }