[pigeon] Fix tool hangs on verbose sub-processes (#6198)

The Pigeon tool hangs on Windows if you don't have the Java formatter on your path. Repro examples:

```
dart ./tool/generate.dart
```

```
dart ./tool/test.dart -f windows_integration_tests
```

The root cause is that the tool runs sub-processes without consuming their stdout/stderr output. The sub-process blocks if these pipes get full. See: https://api.dart.dev/stable/3.3.0/dart-io/Process-class.html

This change is untested. See: #6198 (comment)

Needed for #6196

Part of flutter/flutter#144042

packages/pigeon/tool/shared/process_utils.dart

@@ -16,13 +16,29 @@ Future<int> runProcess(String command, List<String> arguments,
     mode:
         streamOutput ? ProcessStartMode.inheritStdio : ProcessStartMode.normal,
   );
+
+  if (streamOutput) {
+    return process.exitCode;
+  }
+
+  final List<int> stdoutBuffer = <int>[];
+  final List<int> stderrBuffer = <int>[];
+  final Future<void> stdoutFuture = process.stdout.forEach(stdoutBuffer.addAll);
+  final Future<void> stderrFuture = process.stderr.forEach(stderrBuffer.addAll);
   final int exitCode = await process.exitCode;
+  await Future.wait(<Future<void>>[
+    stdoutFuture,
+    stderrFuture,
+  ]);
+
   if (exitCode != 0 && logFailure) {
     // ignore: avoid_print
     print('$command $arguments failed:');
+    stdout.add(stdoutBuffer);
+    stderr.add(stderrBuffer);
     await Future.wait(<Future<void>>[
-      process.stdout.pipe(stdout),
-      process.stderr.pipe(stderr),
+      stdout.flush(),
+      stderr.flush(),
     ]);
   }
   return exitCode;