Only build a full Dart SDK when building for the host system (#8071)

Author: jason-simmons

tools/gn

@@ -232,8 +232,9 @@ def to_gn_args(args):
     if args.arm_float_abi:
       gn_args['arm_float_abi'] = args.arm_float_abi
 
-    # Switch flutter to full Dart SDK.
-    gn_args['dart_platform_sdk'] = False
+    # Switch flutter to full Dart SDK when building for the host system.
+    if args.target_os is None:
+      gn_args['dart_platform_sdk'] = False
 
     return gn_args