[dartdevc] Update URI for libraries.json to use multi-root

I confirmed that this prevents the local paths from leaking into the ddc_sdk.dill
file created by this build target.

Change-Id: I777482f5aea6d70106072f070ab48233ec302682
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/122178
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

Author: nshahan

utils/dartdevc/BUILD.gn

@@ -372,9 +372,9 @@ prebuilt_dart_action("dartdevc_kernel_sdk_outline") {
     "--multi-root-scheme",
     "org-dartlang-sdk",
     "--multi-root",
-    rebase_path(sdk_root),
+    "file:///" + rebase_path(sdk_root),
     "--libraries-file",
-    "file:///" + rebase_path("$sdk_root/lib/libraries.json"),
+    "org-dartlang-sdk:///lib/libraries.json",
     "--output",
     rebase_path(sdk_dill),
     "--source",