Mark the Script.source field as nullable.

Change-Id: I1f176c7aa75eb9a15d3e237d894395632830291e
Reviewed-on: https://dart-review.googlesource.com/c/77981
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>
diff --git a/runtime/vm/service/service.md b/runtime/vm/service/service.md
index ebdb1af..b65f60f 100644
--- a/runtime/vm/service/service.md
+++ b/runtime/vm/service/service.md
@@ -2372,9 +2372,9 @@
   // The library which owns this script.
   @Library library;
 
-  // The source code for this script. For certain built-in scripts,
-  // this may be reconstructed without source comments.
-  string source;
+  // The source code for this script. This can be null for certain built-in
+  // scripts.
+  string source [optional];
 
   // A table encoding a mapping from token position to line and column.
   int[][] tokenPosTable;
diff --git a/runtime/vm/service/service_dev.md b/runtime/vm/service/service_dev.md
index e2dfbc4..39d695a 100644
--- a/runtime/vm/service/service_dev.md
+++ b/runtime/vm/service/service_dev.md
@@ -2372,9 +2372,9 @@
   // The library which owns this script.
   @Library library;
 
-  // The source code for this script. For certain built-in scripts,
-  // this may be reconstructed without source comments.
-  string source;
+  // The source code for this script. This can be null for certain built-in
+  // scripts.
+  string source [optional];
 
   // A table encoding a mapping from token position to line and column.
   int[][] tokenPosTable;