libtxt: add missing dependency for Mac/iOS builds (#4931)

third_party/txt/BUILD.gn

@@ -103,16 +103,6 @@ source_set("txt") {
     "src/utils/WindowsUtils.h",
   ]
 
-  if (is_mac || is_ios) {
-    set_sources_assignment_filter([])
-    sources += [ "src/txt/platform_mac.mm" ]
-    set_sources_assignment_filter(sources_assignment_filter)
-  } else if (is_android) {
-    sources += [ "src/txt/platform_android.cc" ]
-  } else {
-    sources += [ "src/txt/platform.cc" ]
-  }
-
   public_configs = [ ":txt_config" ]
 
   public_deps = [
@@ -125,6 +115,17 @@ source_set("txt") {
   deps = [
     "//third_party/skia:effects",
   ]
+
+  if (is_mac || is_ios) {
+    set_sources_assignment_filter([])
+    sources += [ "src/txt/platform_mac.mm" ]
+    set_sources_assignment_filter(sources_assignment_filter)
+    deps += [ "$flutter_root/fml" ]
+  } else if (is_android) {
+    sources += [ "src/txt/platform_android.cc" ]
+  } else {
+    sources += [ "src/txt/platform.cc" ]
+  }
 }
 
 txt_common_executable_deps = [