add version to pubspec.yaml (#16857)

Uses the `version` property from the `pubspec.yaml` file to set the corresponding fields in the `local.properties` file respectively in the `Generated.xcconfig` file.

The `--build-name` and `--build-number` options have changed. Now they trump the `version` property from the `pubspec.yaml` file.

If the `version` property is not set and the  `--build-name` and `--build-number` options are not provided, the build command will not change the `local.properties` / `Generated.xcconfig` file.

packages/flutter_tools/test/project_test.dart

diff --git a/packages/flutter_tools/test/project_test.dart b/packages/flutter_tools/test/project_test.dart
index 632018d..8955187 100644
--- a/packages/flutter_tools/test/project_test.dart
+++ b/packages/flutter_tools/test/project_test.dart
@@ -20,23 +20,23 @@
     group('ensure ready for platform-specific tooling', () {
       testInMemory('does nothing, if project is not created', () async {
         final FlutterProject project = someProject();
-        project.ensureReadyForPlatformSpecificTooling();
+        await project.ensureReadyForPlatformSpecificTooling();
         expect(project.directory.existsSync(), isFalse);
       });
       testInMemory('does nothing in plugin or package root project', () async {
         final FlutterProject project = aPluginProject();
-        project.ensureReadyForPlatformSpecificTooling();
+        await project.ensureReadyForPlatformSpecificTooling();
         expect(project.ios.directory.childFile('Runner/GeneratedPluginRegistrant.h').existsSync(), isFalse);
         expect(project.ios.directory.childFile('Flutter/Generated.xcconfig').existsSync(), isFalse);
       });
       testInMemory('injects plugins', () async {
         final FlutterProject project = aProjectWithIos();
-        project.ensureReadyForPlatformSpecificTooling();
+        await project.ensureReadyForPlatformSpecificTooling();
         expect(project.ios.directory.childFile('Runner/GeneratedPluginRegistrant.h').existsSync(), isTrue);
       });
       testInMemory('generates Xcode configuration', () async {
         final FlutterProject project = aProjectWithIos();
-        project.ensureReadyForPlatformSpecificTooling();
+        await project.ensureReadyForPlatformSpecificTooling();
         expect(project.ios.directory.childFile('Flutter/Generated.xcconfig').existsSync(), isTrue);
       });
     });