[dartdevc] don't gen source maps for helper code compiled with dart2js

Change-Id: If1108ce6f8adf6673abeaafe8f060833284b516d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106020
Commit-Queue: Jake Macdonald <jakemac@google.com>
Auto-Submit: Vijay Menon <vsm@google.com>
Reviewed-by: Jake Macdonald <jakemac@google.com>

utils/dartdevc/BUILD.gn

@@ -87,6 +87,7 @@ template("dart2js_compile") {
       "$abs_main",
       "-m",
       "-o$abs_output",
+      "--no-source-maps",
     ]
   }
 }