Android projects with 'apply plugin: "com.android.library"' now fail to build

[jtreanor]

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Environment

Run react-native info in your terminal and paste its contents here.

React Native Environment Info:
    System:
      OS: macOS 10.14.1
      CPU: (12) x64 Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
      Memory: 279.43 MB / 32.00 GB
      Shell: 5.3 - /bin/zsh
    Binaries:
      Node: 10.12.0 - ~/.nvm/versions/node/v10.12.0/bin/node
      Yarn: 1.10.1 - /usr/local/bin/yarn
      npm: 6.4.1 - ~/.nvm/versions/node/v10.12.0/bin/npm
      Watchman: 4.9.0 - /usr/local/bin/watchman
    SDKs:
      iOS SDK:
        Platforms: iOS 12.1, macOS 10.14, tvOS 12.1, watchOS 5.1
      Android SDK:
        API Levels: 23, 24, 26, 27, 28
        Build Tools: 27.0.0, 27.0.3, 28.0.2, 28.0.3
        System Images: android-27 | Google APIs Intel x86 Atom, android-28 | Google Play Intel x86 Atom
    IDEs:
      Android Studio: 3.2 AI-181.5540.7.32.5056338
      Xcode: 10.1/10B61 - /usr/bin/xcodebuild
    npmPackages:
      react: 16.6.1 => 16.6.1
      react-native: 0.57.5 => 0.57.5
    npmGlobalPackages:
      react-native-cli: 2.0.1
      react-native-create-library: 3.1.2

Description

Any Android React Native project which uses apply plugin: "com.android.library" currently fails to build. The problem is that a recent change (#20526) to react.gradle now uses android.applicationVariants when configuring the project. This is not present for library projects, so it fails.

Prior to this changes, Android library projects could be build successfully, so this is a regression.

Reproducible Demo

I have put a simple project to demonstrate the issue here: https://github.com/jtreanor/sample-library-react-native.

This project is the generated content of react-native init SampleLibrary with the only change being modifying build.gradle to add apply plugin: "com.android.library"

The following steps will reproduce the error:

1. git clone git@github.com:jtreanor/sample-library-react-native.git
2. cd sample-library-react-native
3. yarn install
4. cd android && ./gradlew build

You will see an error that looks something like this:

FAILURE: Build failed with an exception.

* Where:
Script '/Users/james/src/SampleLibrary/node_modules/react-native/react.gradle' line: 15

* What went wrong:
A problem occurred configuring project ':app'.
> Could not get unknown property 'applicationVariants' for object of type com.android.build.gradle.LibraryExtension.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 0s