[script] Remove unnecessary breaks in default clauses of switch statements (#7469)

See
https://dart.googlesource.com/sdk.git/+/045d26bc74209f5acc6466669f89686344e83de2

script/tool/lib/src/native_test_command.dart

@@ -500,7 +500,6 @@ this command.
           if (exampleHasUnitTests) {
             ranUnitTests = true;
           }
-          break;
       }
     }
 

script/tool/lib/src/update_excerpts_command.dart

@@ -149,7 +149,6 @@ class UpdateExcerptsCommand extends PackageLoopingCommand {
                     language = 'groovy';
                   default:
                     language = extension.substring(1);
-                    break;
                 }
                 final String section = match.namedGroup('section')!;
                 final String plaster = match.namedGroup('plaster') ?? '···';

script/tool/test/util.dart

@@ -313,7 +313,6 @@ String _pluginPlatformSection(
         ]);
       default:
         assert(false, 'Unrecognized platform: $platform');
-        break;
     }
     entry = '${lines.join('\n')}\n';
   }