[flutter_tools] ensure --dart-define can parse args with commas (#73798)

dev/bots/test.dart

@@ -1028,13 +1028,13 @@ Future<void> _runWebIntegrationTests() async {
   await _runWebDebugTest('lib/null_safe_main.dart', enableNullSafety: true);
   await _runWebDebugTest('lib/web_define_loading.dart',
     additionalArguments: <String>[
-      '--dart-define=test.valueA=Example',
+      '--dart-define=test.valueA=Example,A',
       '--dart-define=test.valueB=Value',
     ]
   );
   await _runWebReleaseTest('lib/web_define_loading.dart',
     additionalArguments: <String>[
-      '--dart-define=test.valueA=Example',
+      '--dart-define=test.valueA=Example,A',
       '--dart-define=test.valueB=Value',
     ]
   );

dev/devicelab/lib/tasks/integration_tests.dart

@@ -131,7 +131,7 @@ TaskFunction dartDefinesTask() {
   return DriverTest(
     '${flutterDirectory.path}/dev/integration_tests/ui',
     'lib/defines.dart', extraOptions: <String>[
-    '--dart-define=test.valueA=Example',
+    '--dart-define=test.valueA=Example,A',
     '--dart-define=test.valueB=Value',
     ],
   );

dev/integration_tests/ui/test_driver/defines_test.dart

@@ -17,6 +17,6 @@ void main() {
   });
 
   test('Can run with --dart-define', () async {
-    await driver.waitFor(find.text('ExampleValue'));
+    await driver.waitFor(find.text('Example,AValue'));
   });
 }

dev/integration_tests/web/lib/web_define_loading.dart

@@ -8,7 +8,7 @@ Future<void> main() async {
   final StringBuffer output = StringBuffer();
   const String combined = String.fromEnvironment('test.valueA') +
     String.fromEnvironment('test.valueB');
-  if (combined == 'ExampleValue') {
+  if (combined == 'Example,AValue') {
     output.write('--- TEST SUCCEEDED ---');
     print('--- TEST SUCCEEDED ---');
   } else {

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -476,6 +476,7 @@ abstract class FlutterCommand extends Command<void> {
             'and double.fromEnvironment constructors.\n'
             'Multiple defines can be passed by repeating --dart-define multiple times.',
       valueHelp: 'foo=bar',
+      splitCommas: false,
     );
   }