flutter build apk --releaseのときだけ、以下のエラーが出てapkが作れない問題でハマりました。

FAILURE: Build completed with 2 failures.

1: Task failed with an exception.
-----------
* What went wrong:
Execution failed for task ':generateReleaseRFile'.
> Could not resolve all files for configuration ':releaseRuntimeClasspath'.
   > Could not find com.google.firebase:firebase-bom:25.12.0.
     Searched in the following locations:
       - https://storage.googleapis.com/download.flutter.io/com/google/firebase/firebase-bom/25.12.0/firebase-bom-25.12.0.pom
       - https://storage.googleapis.com/download.flutter.io/com/google/firebase/firebase-bom/25.12.0/firebase-bom-25.12.0.jar
     Required by:
         project :
   > Could not find com.google.firebase:firebase-core:.
     Required by:
         project :
   > Could not find androidx.annotation:annotation:1.1.0.
     Searched in the following locations:
       - https://storage.googleapis.com/download.flutter.io/androidx/annotation/annotation/1.1.0/annotation-1.1.0.pom
       - https://storage.googleapis.com/download.flutter.io/androidx/annotation/annotation/1.1.0/annotation-1.1.0.jar
     Required by:
         project :

* 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.
==============================================================================

2: Task failed with an exception.
-----------
* What went wrong:
Execution failed for task ':javaPreCompileRelease'.
> Could not resolve all files for configuration ':releaseCompileClasspath'.
   > Could not find com.google.firebase:firebase-bom:25.12.0.
     Required by:
         project :
   > Could not find com.google.firebase:firebase-core:.
     Required by:
         project :
   > Could not find androidx.annotation:annotation:1.1.0.
     Required by:
         project :

* 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 2s


The plugin firebase_core could not be built due to the issue above.

解決方法

Stackoverflowを参考に、flutter build apk --profile を一度実行すると、 --release でもビルドできるようになりました。

謎い

追記：本当の解決方法

gradleを4.1.0から3.5.0にしたら解決しました。
使っていたFlutter 1.22.2 で新規プロジェクトを作成したら3.5.0だったので、どうやら安易に変更してはいけないようです。

buildscript {
    dependencies {
        classpath 'com.android.tools.build:gradle:3.5.0'
    }
}