Get the DDK build working with NNBD.

When use_nnbd is set, make the build steps for generating the DDK SDK
summary and JS use the forked SDK directory and enable the non-nullable
experiment flag.

Change-Id: Ia483a621dccd40245cdf5e2c1f7fd86b9bc1e266
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/122162
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Bob Nystrom <rnystrom@google.com>

Author: munificent

utils/dartdevc/BUILD.gn

@@ -349,14 +349,19 @@ prebuilt_dart_action("dartdevc_kernel_sdk_outline") {
     "--multi-root-scheme",
     "org-dartlang-sdk",
     "--multi-root",
-    rebase_path("../../sdk"),
+    rebase_path(sdk_root),
     "--libraries-file",
     "file:///" + rebase_path("$sdk_root/lib/libraries.json"),
     "--output",
     rebase_path(sdk_dill),
     "--source",
     "dart:core",
   ]
+
+  # TODO(38701): Cleanup after merging the forked SDK into mainline.
+  if (use_nnbd) {
+    args += [ "--enable-experiment=non-nullable" ]
+  }
 }
 
 copy("dartdevc_kernel_sdk_libraries_json") {
@@ -424,4 +429,9 @@ prebuilt_dart_action("dartdevc_kernel_sdk") {
     "-o",
     rebase_path("$target_gen_dir/kernel/legacy/dart_sdk.js"),
   ]
+
+  # TODO(38701): Cleanup after merging the forked SDK into mainline.
+  if (use_nnbd) {
+    args += [ "--enable-experiment=non-nullable" ]
+  }
 }