Flutter実行時のエラー`Execution failed for task ':app:mergeDexDebug'.`の対応

FirebaseやFirestoreのパッケージを入れてRunを押したときに下記エラーが発生。

Launching lib/main.dart on Android SDK built for x86 in debug mode...
Running Gradle task 'assembleDebug'...
Checking the license for package Android SDK Tools in /Users/kazufumisuzuki/Library/Android/sdk/licenses
License for package Android SDK Tools accepted.
Preparing "Install Android SDK Tools (revision: 26.1.1)".
"Install Android SDK Tools (revision: 26.1.1)" ready.
Installing Android SDK Tools in /Users/kazufumisuzuki/Library/Android/sdk/tools
"Install Android SDK Tools (revision: 26.1.1)" complete.
"Install Android SDK Tools (revision: 26.1.1)" finished.
Checking the license for package Android SDK Build-Tools 29.0.2 in /Users/kazufumisuzuki/Library/Android/sdk/licenses
License for package Android SDK Build-Tools 29.0.2 accepted.
Preparing "Install Android SDK Build-Tools 29.0.2 (revision: 29.0.2)".
"Install Android SDK Build-Tools 29.0.2 (revision: 29.0.2)" ready.
Installing Android SDK Build-Tools 29.0.2 in /Users/kazufumisuzuki/Library/Android/sdk/build-tools/29.0.2
"Install Android SDK Build-Tools 29.0.2 (revision: 29.0.2)" complete.
"Install Android SDK Build-Tools 29.0.2 (revision: 29.0.2)" finished.
注意:/Users/kazufumisuzuki/Developer/flutter/.pub-cache/hosted/pub.dartlang.org/cloud_firestore-0.13.7/android/src/main/java/io/flutter/plugins/firebase/cloudfirestore/CloudFirestorePlugin.javaの操作は、未チェックまたは安全ではありません。
注意:詳細は、-Xlint:uncheckedオプションを指定して再コンパイルしてください。
D8: Cannot fit requested classes in a single dex file (# methods: 87322 > 65536)
com.android.builder.dexing.DexArchiveMergerException: Error while merging dex archives: 
The number of method references in a .dex file cannot exceed 64K.
Learn how to resolve this issue at https://developer.android.com/tools/building/multidex.html
	at com.android.builder.dexing.D8DexArchiveMerger.getExceptionToRethrow(D8DexArchiveMerger.java:132)
	at com.android.builder.dexing.D8DexArchiveMerger.mergeDexArchives(D8DexArchiveMerger.java:119)
	at com.android.build.gradle.internal.transforms.DexMergerTransformCallable.call(DexMergerTransformCallable.java:102)
	at com.android.build.gradle.internal.tasks.DexMergingTaskRunnable.run(DexMergingTask.kt:441)
	at com.android.build.gradle.internal.tasks.Workers$ActionFacade.run(Workers.kt:242)
	at org.gradle.workers.internal.AdapterWorkAction.execute(AdapterWorkAction.java:50)
	at org.gradle.workers.internal.DefaultWorkerServer.execute(DefaultWorkerServer.java:50)
	at org.gradle.workers.internal.NoIsolationWorkerFactory$1$1.create(NoIsolationWorkerFactory.java:63)
	at org.gradle.workers.internal.NoIsolationWorkerFactory$1$1.create(NoIsolationWorkerFactory.java:59)
	at org.gradle.internal.classloader.ClassLoaderUtils.executeInClassloader(ClassLoaderUtils.java:98)
	at org.gradle.workers.internal.NoIsolationWorkerFactory$1.lambda$execute$0(NoIsolationWorkerFactory.java:59)
	at org.gradle.workers.internal.AbstractWorker$1.call(AbstractWorker.java:44)
	at org.gradle.workers.internal.AbstractWorker$1.call(AbstractWorker.java:41)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor$CallableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:416)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor$CallableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:406)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor$1.execute(DefaultBuildOperationExecutor.java:165)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:250)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:158)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor.call(DefaultBuildOperationExecutor.java:102)
	at org.gradle.internal.operations.DelegatingBuildOperationExecutor.call(DelegatingBuildOperationExecutor.java:36)
	at org.gradle.workers.internal.AbstractWorker.executeWrappedInBuildOperation(AbstractWorker.java:41)
	at org.gradle.workers.internal.NoIsolationWorkerFactory$1.execute(NoIsolationWorkerFactory.java:53)
	at org.gradle.workers.internal.DefaultWorkerExecutor.lambda$submitWork$2(DefaultWorkerExecutor.java:200)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at org.gradle.internal.work.DefaultConditionalExecutionQueue$ExecutionRunner.runExecution(DefaultConditionalExecutionQueue.java:215)
	at org.gradle.internal.work.DefaultConditionalExecutionQueue$ExecutionRunner.runBatch(DefaultConditionalExecutionQueue.java:164)
	at org.gradle.internal.work.DefaultConditionalExecutionQueue$ExecutionRunner.run(DefaultConditionalExecutionQueue.java:131)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:64)
	at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:48)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
	at java.lang.Thread.run(Thread.java:748)
Caused by: com.android.tools.r8.CompilationFailedException: Compilation failed to complete
	at com.android.tools.r8.utils.W.a(:87)
	at com.android.tools.r8.D8.run(:11)
	at com.android.builder.dexing.D8DexArchiveMerger.mergeDexArchives(D8DexArchiveMerger.java:117)
	... 33 more
Caused by: com.android.tools.r8.utils.b: Error: null, Cannot fit requested classes in a single dex file (# methods: 87322 > 65536)
	at Version.fakeStackEntry(Version_2.0.88.java:0)
	at com.android.tools.r8.utils.O0.a(:21)
	at com.android.tools.r8.dex.P.a(:56)
	at com.android.tools.r8.dex.P$h.a(:7)
	at com.android.tools.r8.dex.b.a(:15)
	at com.android.tools.r8.dex.b.b(:24)
	at com.android.tools.r8.D8.d(:92)
	at com.android.tools.r8.D8.b(:1)
	at com.android.tools.r8.utils.W.a(:30)
	... 35 more


FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:mergeDexDebug'.
> A failure occurred while executing com.android.build.gradle.internal.tasks.Workers$ActionFacade
  > com.android.builder.dexing.DexArchiveMergerException: Error while merging dex archives: 
    The number of method references in a .dex file cannot exceed 64K.
    Learn how to resolve this issue at https://developer.android.com/tools/building/multidex.html

* 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 4m 25s
[!] The shrinker may have failed to optimize the Java bytecode.
   To disable the shrinker, pass the `--no-shrink` flag to this command.
   To learn more, see: https://developer.android.com/studio/build/shrink-code
Exception: Gradle task assembleDebug failed with exit code 1

対応方法：app/build.gradleに下記の記載を追加する。

android {
   defaultConfig {
       // ...
       multiDexEnabled true // これを追加する
   }
}

サーバエンジニアとしては何が起こっているかよくわからない。