Exclude third_party/dart/third_party/devtools from the license script

This component will be added to the engine dependencies by
#25912

Author: jason-simmons

ci/licenses_golden/tool_signature

@@ -1,2 +1,2 @@
-Signature: d63f7a9bbfe816cff5a335245a3bac58
+Signature: d165688a88dd53153d8e32bd26d3fa6d
 

tools/licenses/lib/main.dart

@@ -2016,7 +2016,8 @@ class _RepositoryDartThirdPartyDirectory extends _RepositoryGenericThirdPartyDir
 
   @override
   bool shouldRecurse(fs.IoNode entry) {
-    return entry.name != 'drt_resources' // test materials
+    return entry.name != 'devtools' // not linked in
+        && entry.name != 'drt_resources' // test materials
         && entry.name != 'firefox_jsshell' // testing tool for dart2js
         && entry.name != 'd8' // testing tool for dart2js
         && entry.name != 'pkg'