[pigeon] Eliminate some of the test pigeons (flutter#3213)

* Fold in async_handlers.dart

* Remove some largely unused files

* Fold in the relevant parts of list.dart

* Remove java_double_host_api

* Remove unnecessary double-generation of messages.dart

* Remove enum_args

* Remove some unnecesasry/duplicate unit tests

* Eliminate android_unittest

* Format

* Remove build references on Windows

Author: stuartmorgan-g

packages/pigeon/pigeons/async_handlers.dart

@@ -538,10 +538,68 @@ abstract class FlutterIntegrationCoreApi {
   @ObjCSelector('echoNullableMap:')
   @SwiftFunction('echoNullable(_:)')
   Map<String?, Object?>? echoNullableMap(Map<String?, Object?>? aMap);
+
+  // ========== Async tests ==========
+  // These are minimal since async FlutterApi only changes Dart generation.
+  // Currently they aren't integration tested, but having them here ensures
+  // analysis coverage.
+
+  /// A no-op function taking no arguments and returning no value, to sanity
+  /// test basic asynchronous calling.
+  @async
+  void noopAsync();
+
+  /// Returns the passed in generic Object asynchronously.
+  @async
+  @ObjCSelector('echoAsyncString:')
+  @SwiftFunction('echoAsync(_:)')
+  String echoAsyncString(String aString);
 }
 
 /// An API that can be implemented for minimal, compile-only tests.
+//
+// This is also here to test that multiple host APIs can be generated
+// successfully in all languages (e.g., in Java where it requires having a
+// wrapper class).
 @HostApi()
 abstract class HostTrivialApi {
   void noop();
 }
+
+/// A simple API implemented in some unit tests.
+//
+// This is separate from HostIntegrationCoreApi to avoid having to update a
+// lot of unit tests every time we add something to the integration test API.
+// TODO(stuartmorgan): Restructure the unit tests to reduce the number of
+// different APIs we define.
+@HostApi()
+abstract class HostSmallApi {
+  @async
+  @ObjCSelector('echoString:')
+  String echo(String aString);
+
+  @async
+  void voidVoid();
+}
+
+/// A simple API called in some unit tests.
+//
+// This is separate from FlutterIntegrationCoreApi to allow for incrementally
+// moving from the previous fragmented unit test structure to something more
+// unified.
+// TODO(stuartmorgan): Restructure the unit tests to reduce the number of
+// different APIs we define.
+@FlutterApi()
+abstract class FlutterSmallApi {
+  @ObjCSelector('echoWrappedList:')
+  @SwiftFunction('echo(_:)')
+  TestMessage echoWrappedList(TestMessage msg);
+}
+
+/// A data class containing a List, used in unit tests.
+// TODO(stuartmorgan): Evaluate whether these unit tests are still useful; see
+// TODOs above about restructring.
+class TestMessage {
+  // ignore: always_specify_types, strict_raw_type
+  List? testList;
+}