Document single root flag

Change-Id: Ibccbf1346bdacaa6cda5ae1b0bbe479bc8cc1164
Reviewed-on: https://dart-review.googlesource.com/c/78821
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Peter von der Ahé <ahe@google.com>
diff --git a/pkg/front_end/lib/src/fasta/fasta_codes_generated.dart b/pkg/front_end/lib/src/fasta/fasta_codes_generated.dart
index 18949ae..e6e32f4 100644
--- a/pkg/front_end/lib/src/fasta/fasta_codes_generated.dart
+++ b/pkg/front_end/lib/src/fasta/fasta_codes_generated.dart
@@ -3382,6 +3382,17 @@
   --supermixin
     Ignored for now.
 
+  --single-root-scheme=String
+  --single-root-base=<dir>
+    Specify a custom URI scheme and a location on disk where such URIs are
+    mapped to.
+
+    When specified, the compiler can be invoked with inputs using the custom
+    URI scheme. The compiler can ignore the exact location of files on disk
+    and as a result to produce output that is independendent of the absolute
+    location of files on disk. This is mostly useful for integrating with
+    build systems.
+
   --fatal=errors
   --fatal=warnings
     Makes messages of the given kinds fatal, that is, immediately stop the
diff --git a/pkg/front_end/messages.yaml b/pkg/front_end/messages.yaml
index 609672d..d8c6e49 100644
--- a/pkg/front_end/messages.yaml
+++ b/pkg/front_end/messages.yaml
@@ -1520,6 +1520,17 @@
       --supermixin
         Ignored for now.
 
+      --single-root-scheme=String
+      --single-root-base=<dir>
+        Specify a custom URI scheme and a location on disk where such URIs are
+        mapped to.
+
+        When specified, the compiler can be invoked with inputs using the custom
+        URI scheme. The compiler can ignore the exact location of files on disk
+        and as a result to produce output that is independendent of the absolute
+        location of files on disk. This is mostly useful for integrating with
+        build systems.
+
       --fatal=errors
       --fatal=warnings
         Makes messages of the given kinds fatal, that is, immediately stop the