[gardening] Fix NNDB flag use for Flutter HHH build.

Our //build/config/BUILDCONFIG.gn isn't used for Flutter, so we must
put the default version where the Flutter build can see it.

Change-Id: I99afc99209b3721c48aa56ef413910d34df1bb8c
Cq-Include-Trybots: luci.dart.try:flutter-engine-linux-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/120580
Reviewed-by: Teagan Strickland <sstrickl@google.com>
Commit-Queue: Teagan Strickland <sstrickl@google.com>

BUILD.gn

@@ -6,6 +6,12 @@ import("build/config/gclient_args.gni")
 
 targetting_fuchsia = target_os == "fuchsia"
 
+declare_args() {
+  # Whether to use the NNBD fork of the SDK core libraries.
+  # TODO(rnystrom): Remove this when the fork has been merged back in.
+  use_nnbd = false
+}
+
 # This target will be built if no target is specified when invoking ninja.
 group("default") {
   if (targetting_fuchsia) {

build/config/BUILDCONFIG.gn

@@ -130,10 +130,6 @@ declare_args() {
 
   # Compile for Thread Sanitizer to find threading bugs.
   is_tsan = false
-
-  # Whether to use the NNBD fork of the SDK core libraries.
-  # TODO(rnystrom): Remove this when the fork has been merged back in.
-  use_nnbd = false
 }
 
 # =============================================================================

utils/dartdevc/BUILD.gn

@@ -207,13 +207,8 @@ group("dartdevc_test") {
     ":dartdevc_kernel_sdk",
     ":dartdevc_sdk",
     ":dartdevc_test_pkg",
+    "../..:create_sdk",
   ]
-
-  if (use_nnbd) {
-    deps += [ "../../sdk_nnbd:create_sdk" ]
-  } else {
-    deps += [ "../../sdk:create_sdk" ]
-  }
 }
 
 # Builds everything needed to run dartdevc tests locally using test.dart without