[fuchsia] Update Fuchsia API level to 18

Bug: b/339619418
Change-Id: Id8cc841ca1efd4356d54ed5513e958dc2c8c0cc6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/365981
Reviewed-by: Ryan Macnak <rmacnak@google.com>
diff --git a/tools/gn.py b/tools/gn.py
index 11e88fd..5e7b713 100755
--- a/tools/gn.py
+++ b/tools/gn.py
@@ -235,7 +235,7 @@
             gn_args['arm_float_abi'] = floatabi
             gn_args['arm_use_neon'] = True
     if gn_args['target_os'] == 'fuchsia':
-        gn_args['fuchsia_target_api_level'] = 16
+        gn_args['fuchsia_target_api_level'] = 18
 
     gn_args['is_debug'] = mode == 'debug'
     gn_args['is_release'] = mode == 'release'