[pigeon] Consolidate mock handler tests (flutter#4642)

Eliminates the `mock_handler_tester` sub-package, which existed just for a single unit test file and required duplicate `pigeons/message.dart` output generation with bespoke test wrappers, folding that test into the shared harness and generation system. This also eliminates an entire test suite from the test harness since it's now part of the rest of the Dart unit testing.

This surfaced an edge-case problem with the new package name override, which is that if the override doesn't match the actual dart output location, generated Dart test output won't compile because the import will be wrong. This addresses that by passing the actual package as well to the test generator.

Author: stuartmorgan-g

packages/pigeon/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 10.1.5
+
+* Fixes import in generated Dart test output when overriding package name.
+
 ## 10.1.4
 
 * Adds package name to method channel strings to avoid potential collisions between plugins.

packages/pigeon/lib/dart_generator.dart

@@ -599,6 +599,7 @@ if (replyList == null) {
     Root root,
     StringSink sink, {
     required String dartPackageName,
+    required String dartOutputPackageName,
   }) {
     final Indent indent = Indent(sink);
     final String sourceOutPath = generatorOptions.sourceOutPath ?? '';
@@ -620,7 +621,7 @@ if (replyList == null) {
     } else {
       final String path =
           relativeDartPath.replaceFirst(RegExp(r'^.*/lib/'), '');
-      indent.writeln("import 'package:$dartPackageName/$path';");
+      indent.writeln("import 'package:$dartOutputPackageName/$path';");
     }
     for (final Api api in root.apis) {
       if (api.location == ApiLocation.host && api.dartHostTestHandler != null) {

packages/pigeon/lib/generator_tools.dart

@@ -13,7 +13,7 @@ import 'ast.dart';
 /// The current version of pigeon.
 ///
 /// This must match the version in pubspec.yaml.
-const String pigeonVersion = '10.1.4';
+const String pigeonVersion = '10.1.5';
 
 /// Read all the content from [stdin] to a String.
 String readStdin() {

packages/pigeon/lib/pigeon_lib.dart

@@ -510,11 +510,16 @@ class DartTestGeneratorAdapter implements GeneratorAdapter {
       basePath: options.basePath ?? '',
     );
     const DartGenerator testGenerator = DartGenerator();
+    // The test code needs the actual package name of the Dart output, even if
+    // the package name has been overridden for other uses.
+    final String outputPackageName =
+        deducePackageName(options.dartOut ?? '') ?? options.getPackageName();
     testGenerator.generateTest(
       dartOptionsWithHeader,
       root,
       sink,
       dartPackageName: options.getPackageName(),
+      dartOutputPackageName: outputPackageName,
     );
   }