Fix missing_return violation newly enforced in Dart ~2.3.2-dev.0.1 (#1970)

Author: srawlins

lib/src/source_linker.dart

@@ -113,6 +113,8 @@ class SourceLinker {
         case '%l%':
           return lineNumber.toString();
           break;
+        default:
+          return null;
       }
     });
   }