Build DDC targets with forked SDK sources when building with --nnbd flag

Issue: #38665

Change-Id: I42eb4e97f3269f7d1a53abeac62f9293a003ae60
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/120400
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>

BUILD.gn

@@ -3,15 +3,10 @@
 # BSD-style license that can be found in the LICENSE file.
 
 import("build/config/gclient_args.gni")
+import("sdk_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) {

sdk_args.gni

@@ -0,0 +1,5 @@
+declare_args() {
+  # Whether to use the NNBD fork of the SDK core libraries.
+  # TODO(#38701): Remove this when the fork has been merged back in.
+  use_nnbd = false
+}

utils/dartdevc/BUILD.gn

@@ -3,6 +3,7 @@
 # BSD-style license that can be found in the LICENSE file.
 
 import("../../build/dart/dart_action.gni")
+import("../../sdk_args.gni")
 import("../application_snapshot.gni")
 import("../create_timestamp.gni")
 
@@ -35,10 +36,17 @@ application_snapshot("dartdevc") {
   ]
 }
 
+# TODO(#38701) Cleanup after merging the forked SDK into mainline.
+if (use_nnbd) {
+  sdk_root = "../../sdk_nnbd"
+} else {
+  sdk_root = "../../sdk"
+}
+
 sdk_lib_files = exec_script("../../tools/list_dart_files.py",
                             [
                               "absolute",
-                              rebase_path("../../sdk/lib"),
+                              rebase_path("$sdk_root/lib"),
                             ],
                             "list lines")
 
@@ -119,7 +127,7 @@ prebuilt_dart_action("dartdevc_patch_sdk") {
   inputs += exec_script("../../tools/list_dart_files.py",
                         [
                           "absolute",
-                          rebase_path("../../sdk/lib/_internal/js_dev_runtime"),
+                          rebase_path("$sdk_root/lib/_internal/js_dev_runtime"),
                         ],
                         "list lines")
 
@@ -132,9 +140,14 @@ prebuilt_dart_action("dartdevc_patch_sdk") {
 
   args = [
     rebase_path("../../"),
-    rebase_path("../../sdk/lib/_internal/js_dev_runtime"),
+    rebase_path("$sdk_root/lib/_internal/js_dev_runtime"),
     rebase_path(patched_sdk_dir),
   ]
+
+  # TODO(#38701) Cleanup after merging the forked SDK into mainline.
+  if (use_nnbd) {
+    args += [ "sdk_nnbd" ]
+  }
 }
 
 # Compiles the Dart core libraries and DDC runtime to an analyzer summary and
@@ -228,7 +241,7 @@ create_timestamp_file("dartdevc_files_stamp") {
 }
 
 create_timestamp_file("dartdevc_sdk_patch_stamp") {
-  path = rebase_path("../../sdk/lib/_internal/js_dev_runtime")
+  path = rebase_path("$sdk_root/lib/_internal/js_dev_runtime")
   output = "$target_gen_dir/dartdevc_sdk_patch_stamp.stamp"
 }
 
@@ -338,7 +351,7 @@ prebuilt_dart_action("dartdevc_kernel_sdk_outline") {
     "--multi-root",
     rebase_path("../../sdk"),
     "--libraries-file",
-    "file:///" + rebase_path("../../sdk/lib/libraries.json"),
+    "file:///" + rebase_path("$sdk_root/lib/libraries.json"),
     "--output",
     rebase_path(sdk_dill),
     "--source",
@@ -348,7 +361,7 @@ prebuilt_dart_action("dartdevc_kernel_sdk_outline") {
 
 copy("dartdevc_kernel_sdk_libraries_json") {
   sources = [
-    rebase_path("../../sdk/lib/libraries.json"),
+    rebase_path("$sdk_root/lib/libraries.json"),
   ]
   outputs = [
     sdk_libraries_json,
@@ -389,7 +402,7 @@ prebuilt_dart_action("dartdevc_kernel_sdk") {
     "--multi-root-scheme",
     "org-dartlang-sdk",
     "--multi-root",
-    "file:///" + rebase_path("../../sdk"),
+    "file:///" + rebase_path(sdk_root),
     "--multi-root-output-path",
     rebase_path("$target_gen_dir/../../"),
     "--libraries-file",