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This relands https://github.com/flutter/flutter/pull/168437. The google3 fixes were landed in: https://github.com/flutter/flutter/pull/171547, https://critique.corp.google.com/cl/781275353, https://github.com/flutter/flutter/pull/171933. This PR is split into two commits: 1. d6253794e8982348c5c21cb63e8f6bf785664be6, code from https://github.com/flutter/flutter/pull/168437 without any changes 2. f35d29e4af630d2d4fdb0cda8686b6ff9f77227a, updates the PR to omit obvious types. Original PR description: ## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: https://github.com/flutter/flutter/pull/158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 3. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example https://github.com/flutter/flutter/pull/168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 4. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps.