Move TaskQueueType to its own file

packages/pigeon/lib/src/java/java_generator.dart

@@ -6,7 +6,7 @@ import '../ast.dart';
 import '../functional.dart';
 import '../generator.dart';
 import '../generator_tools.dart';
-import '../pigeon_lib.dart' show TaskQueueType;
+import '../types/task_queue.dart';
 
 /// Documentation open symbol.
 const String _docCommentPrefix = '/**';

packages/pigeon/lib/src/kotlin/kotlin_generator.dart

@@ -8,7 +8,7 @@ import '../ast.dart';
 import '../functional.dart';
 import '../generator.dart';
 import '../generator_tools.dart';
-import '../pigeon_lib.dart' show TaskQueueType;
+import '../types/task_queue.dart';
 import 'templates.dart';
 
 /// Documentation open symbol.

packages/pigeon/lib/src/objc/objc_generator.dart

@@ -6,7 +6,8 @@ import '../ast.dart';
 import '../functional.dart';
 import '../generator.dart';
 import '../generator_tools.dart';
-import '../pigeon_lib.dart' show Error, TaskQueueType;
+import '../pigeon_lib.dart' show Error;
+import '../types/task_queue.dart';
 
 /// Documentation comment open symbol.
 const String _docCommentPrefix = '///';

packages/pigeon/lib/src/pigeon_lib.dart

@@ -36,6 +36,9 @@ import 'java/java_generator.dart';
 import 'kotlin/kotlin_generator.dart';
 import 'objc/objc_generator.dart';
 import 'swift/swift_generator.dart';
+import 'types/task_queue.dart';
+
+export 'types/task_queue.dart' show TaskQueueType;
 
 class _Asynchronous {
   const _Asynchronous();
@@ -211,21 +214,6 @@ class SwiftClass {
   const SwiftClass();
 }
 
-/// Type of TaskQueue which determines how handlers are dispatched for
-/// HostApi's.
-enum TaskQueueType {
-  /// Handlers are invoked serially on the default thread. This is the value if
-  /// unspecified.
-  serial,
-
-  /// Handlers are invoked serially on a background thread.
-  serialBackgroundThread,
-
-  // TODO(gaaclarke): Add support for concurrent task queues.
-  // /// Handlers are invoked concurrently on a background thread.
-  // concurrentBackgroundThread,
-}
-
 /// Metadata annotation to control how handlers are dispatched for HostApi's.
 /// Note that the TaskQueue API might not be available on the target version of
 /// Flutter, see also:

packages/pigeon/lib/src/swift/swift_generator.dart

@@ -9,7 +9,7 @@ import '../ast.dart';
 import '../functional.dart';
 import '../generator.dart';
 import '../generator_tools.dart';
-import '../pigeon_lib.dart';
+import '../types/task_queue.dart';
 import 'templates.dart';
 
 /// Documentation comment open symbol.

packages/pigeon/lib/src/types/task_queue.dart

@@ -0,0 +1,18 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// Type of TaskQueue which determines how handlers are dispatched for
+/// HostApi's.
+enum TaskQueueType {
+  /// Handlers are invoked serially on the default thread. This is the value if
+  /// unspecified.
+  serial,
+
+  /// Handlers are invoked serially on a background thread.
+  serialBackgroundThread,
+
+  // TODO(gaaclarke): Add support for concurrent task queues.
+  // /// Handlers are invoked concurrently on a background thread.
+  // concurrentBackgroundThread,
+}