Fix Linux plugin template build visibility (#58069)

The CMake plugin build wasn't setting visibility to hidden by default,
which meant that plugins exported everything by default. This would make
bad interactions between plugins much more likely; only the intended API
should be exported by the shared library.

packages/flutter_tools/templates/app/linux.tmpl/CMakeLists.txt.tmpl

@@ -3,6 +3,8 @@ project(runner LANGUAGES CXX)
 
 set(BINARY_NAME "{{projectName}}")
 
+cmake_policy(SET CMP0063 NEW)
+
 set(CMAKE_INSTALL_RPATH "\$ORIGIN")
 
 # Configure build options.

packages/flutter_tools/templates/app/linux.tmpl/flutter/CMakeLists.txt

@@ -65,6 +65,8 @@ add_library(flutter_wrapper_plugin STATIC
 apply_standard_settings(flutter_wrapper_plugin)
 set_target_properties(flutter_wrapper_plugin PROPERTIES
   POSITION_INDEPENDENT_CODE ON)
+set_target_properties(flutter_wrapper_plugin PROPERTIES
+  CXX_VISIBILITY_PRESET hidden)
 target_link_libraries(flutter_wrapper_plugin PUBLIC flutter)
 target_include_directories(flutter_wrapper_plugin PUBLIC
   "${WRAPPER_ROOT}/include"

packages/flutter_tools/templates/plugin/linux.tmpl/CMakeLists.txt.tmpl

@@ -8,6 +8,9 @@ add_library(${PLUGIN_NAME} SHARED
   "${PLUGIN_NAME}.cc"
 )
 apply_standard_settings(${PLUGIN_NAME})
+set_target_properties(${PLUGIN_NAME} PROPERTIES
+  CXX_VISIBILITY_PRESET hidden)
+target_compile_definitions(${PLUGIN_NAME} PRIVATE FLUTTER_PLUGIN_IMPL)
 target_include_directories(${PLUGIN_NAME} INTERFACE
   "${CMAKE_CURRENT_SOURCE_DIR}/include")
 target_link_libraries(${PLUGIN_NAME} PRIVATE flutter flutter_wrapper_plugin)