【Flutter】App requires Multidex supportの治し方。

# 概要
Cloud Firestore導入時に以下のエラーが出ました。
治し方が分かったので備考録程度に書いておきます。
# エラー
```dart
D8: Cannot fit requested classes in a single dex file (# methods: 100585 > 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:432)
	at com.android.build.gradle.internal.tasks.Workers$ActionFacade.run(Workers.kt:242)
	at org.gradle.workers.internal.AdapterWorkAction.execute(AdapterWorkAction.java:57)
	at org.gradle.workers.internal.DefaultWorkerServer.execute(DefaultWorkerServer.java:63)
	at org.gradle.workers.internal.NoIsolationWorkerFactory$1$1.create(NoIsolationWorkerFactory.java:67)
	at org.gradle.workers.internal.NoIsolationWorkerFactory$1$1.create(NoIsolationWorkerFactory.java:63)
	at org.gradle.internal.classloader.ClassLoaderUtils.executeInClassloader(ClassLoaderUtils.java:97)
	at org.gradle.workers.internal.NoIsolationWorkerFactory$1.lambda$execute$0(NoIsolationWorkerFactory.java:63)
	at org.gradle.workers.internal.AbstractWorker$1.call(AbstractWorker.java:44)
	at org.gradle.workers.internal.AbstractWorker$1.call(AbstractWorker.java:41)
	at org.gradle.internal.operations.DefaultBuildOperationRunner$CallableBuildOperationWorker.execute(DefaultBuildOperationRunner.java:200)
	at org.gradle.internal.operations.DefaultBuildOperationRunner$CallableBuildOperationWorker.execute(DefaultBuildOperationRunner.java:195)
	at org.gradle.internal.operations.DefaultBuildOperationRunner$3.execute(DefaultBuildOperationRunner.java:75)
	at org.gradle.internal.operations.DefaultBuildOperationRunner$3.execute(DefaultBuildOperationRunner.java:68)
	at org.gradle.internal.operations.DefaultBuildOperationRunner.execute(DefaultBuildOperationRunner.java:153)
	at org.gradle.internal.operations.DefaultBuildOperationRunner.execute(DefaultBuildOperationRunner.java:68)
	at org.gradle.internal.operations.DefaultBuildOperationRunner.call(DefaultBuildOperationRunner.java:62)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor.lambda$call$2(DefaultBuildOperationExecutor.java:76)
	at org.gradle.internal.operations.UnmanagedBuildOperationWrapper.callWithUnmanagedSupport(UnmanagedBuildOperationWrapper.java:54)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor.call(DefaultBuildOperationExecutor.java:76)
	at org.gradle.workers.internal.AbstractWorker.executeWrappedInBuildOperation(AbstractWorker.java:41)
	at org.gradle.workers.internal.NoIsolationWorkerFactory$1.execute(NoIsolationWorkerFactory.java:60)
	at org.gradle.workers.internal.DefaultWorkerExecutor.lambda$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$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
	at java.lang.Thread.run(Thread.java:748)
Caused by: com.android.tools.r8.CompilationFailedException: Compilation failed to complete, position: null
	at Version.fakeStackEntry(Version_2.1.62.java:0)
	at com.android.tools.r8.utils.Y.a(SourceFile:78)
	at com.android.tools.r8.D8.run(D8.java:11)
	at com.android.builder.dexing.D8DexArchiveMerger.mergeDexArchives(D8DexArchiveMerger.java:117)
	... 36 more
Caused by: com.android.tools.r8.utils.b: Cannot fit requested classes in a single dex file (# methods: 100585 > 65536)
	at com.android.tools.r8.utils.T0.error(SourceFile:1)
	at com.android.tools.r8.utils.T0.a(SourceFile:2)
	at com.android.tools.r8.dex.P.a(SourceFile:740)
	at com.android.tools.r8.dex.P$h.a(SourceFile:7)
	at com.android.tools.r8.dex.b.a(SourceFile:14)
	at com.android.tools.r8.dex.b.b(SourceFile:25)
	at com.android.tools.r8.D8.d(D8.java:133)
	at com.android.tools.r8.D8.b(D8.java:1)
	at com.android.tools.r8.utils.Y.a(SourceFile:36)
	... 38 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 52s
[!] App requires Multidex support
    Flutter multidex handling is disabled. If you wish to let the tool configure multidex, use the --mutidex flag.
Exception: Gradle task assembleDebug failed with exit code 1
```
# 治し方
`project/android/app/build.gradle`内の二箇所を修正しました。
```dart
defaultConfig {
       applicationId "com.****.***"
       minSdkVersion **
       targetSdkVersion **
       versionCode **
       versionName "*.**"

       multiDexEnabled true //追加
 
   }
```
```dart
dependencies {
    implementation 'com.android.support:multidex:2.0.1'//追加
}
```
# 参考文献
https://aqlier.com/2018/06/12/android_cannot-fit/