[pigeon] skipGen (#4682)

adds ability to run pigeon tests without regenerating code (to make iterating and testing easier).

Author: tarrinneal

packages/pigeon/tool/shared/test_runner.dart

@@ -13,17 +13,22 @@ import 'test_suites.dart';
 
 /// Runs the given tests, printing status and exiting with failure if any of
 /// them fails.
-Future<void> runTests(List<String> testsToRun) async {
-  // Pre-generate the necessary common output files.
-  // TODO(stuartmorgan): Consider making this conditional on the specific
-  // tests being run, as not all of them need these files.
-  final String baseDir = p.dirname(p.dirname(Platform.script.toFilePath()));
-  print('# Generating platform_test/ output...');
-  final int generateExitCode = await generateTestPigeons(baseDir: baseDir);
-  if (generateExitCode == 0) {
-    print('Generation complete!');
-  } else {
-    print('Generation failed; see above for errors.');
+Future<void> runTests(
+  List<String> testsToRun, {
+  bool runGeneration = true,
+}) async {
+  if (runGeneration) {
+    // Pre-generate the necessary common output files.
+    // TODO(stuartmorgan): Consider making this conditional on the specific
+    // tests being run, as not all of them need these files.
+    final String baseDir = p.dirname(p.dirname(Platform.script.toFilePath()));
+    print('# Generating platform_test/ output...');
+    final int generateExitCode = await generateTestPigeons(baseDir: baseDir);
+    if (generateExitCode == 0) {
+      print('Generation complete!');
+    } else {
+      print('Generation failed; see above for errors.');
+    }
   }
 
   for (final String test in testsToRun) {

packages/pigeon/tool/test.dart

@@ -18,11 +18,14 @@ import 'shared/test_runner.dart';
 import 'shared/test_suites.dart';
 
 const String _testFlag = 'test';
+const String _noGen = 'no-generation';
 const String _listFlag = 'list';
 
 Future<void> main(List<String> args) async {
   final ArgParser parser = ArgParser()
     ..addMultiOption(_testFlag, abbr: 't', help: 'Only run specified tests.')
+    ..addFlag(_noGen,
+        abbr: 'g', help: 'Skips the generation step.', negatable: false)
     ..addFlag(_listFlag,
         negatable: false, abbr: 'l', help: 'List available tests.')
     ..addFlag('help',
@@ -105,5 +108,5 @@ ${parser.usage}''');
     }
   }
 
-  await runTests(testsToRun);
+  await runTests(testsToRun, runGeneration: !argResults.wasParsed(_noGen));
 }