Move test support logic out of the default Dart output. (#246)

This will allow us to move the mock logic out of the core framework.

Author: Hixie

packages/pigeon/README.md

@@ -23,7 +23,8 @@ doesn't need to worry about conflicting versions of Pigeon.
 
 1) Add Pigeon as a dev_dependency.
 1) Make a ".dart" file outside of your "lib" directory for defining the communication interface.
-1) Run pigeon on your ".dart" file to generate the required Dart and Objective-C code.
+1) Run pigeon on your ".dart" file to generate the required Dart and Objective-C code:
+   `flutter pub get` then `flutter pub run pigeon` with suitable arguments.
 1) Add the generated Dart code to `lib` for compilation.
 1) Add the generated Objective-C code to your Xcode project for compilation
    (e.g. `ios/Runner.xcworkspace` or `.podspec`).
@@ -36,6 +37,7 @@ doesn't need to worry about conflicting versions of Pigeon.
 1) Add Pigeon as a dev_dependency.
 1) Make a ".dart" file outside of your "lib" directory for defining the communication interface.
 1) Run pigeon on your ".dart" file to generate the required Dart and Java code.
+   `flutter pub get` then `flutter pub run pigeon` with suitable arguments.
 1) Add the generated Dart code to `./lib` for compilation.
 1) Add the generated Java code to your `./android/app/src/main/java` directory for compilation.
 1) Implement the generated Java interface for handling the calls on Android, set it up

packages/pigeon/bin/pigeon.dart

@@ -1,7 +1,9 @@
 // Copyright 2020 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.
+
 // @dart = 2.2
+
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
@@ -26,7 +28,9 @@ String _posixRelative(String input, {String from}) {
 
 Future<void> main(List<String> args) async {
   final PigeonOptions opts = Pigeon.parseArgs(args);
-  final Directory tempDir = Directory.systemTemp.createTempSync();
+  final Directory tempDir = Directory.systemTemp.createTempSync(
+    'flutter_pigeon.',
+  );
 
   String importLine = '';
   if (opts.input != null) {
@@ -43,7 +47,6 @@ void main(List<String> args, SendPort sendPort) async {
   sendPort.send(await Pigeon.run(args));
 }
 """;
-
   final File tempFile = File(path.join(tempDir.path, '_pigeon_temp_.dart'));
   await tempFile.writeAsString(code);
   final ReceivePort receivePort = ReceivePort();

packages/pigeon/lib/ast.dart

@@ -32,7 +32,7 @@ class Method extends Node {
   bool isAsynchronous;
 }
 
-/// Represents a collection of [Method]s that are hosted ona given [location].
+/// Represents a collection of [Method]s that are hosted on a given [location].
 class Api extends Node {
   /// Parametric constructor for [Api].
   Api({this.name, this.location, this.methods, this.dartHostTestHandler});