Marcono 1234的回答为我指明了正确的方向。在app/build.gradle中添加了以下代码：

buildTypes {
    release {
        // TODO: Add your own signing config for the release build.
        // Signing with the debug keys for now, so `flutter run --release` works.
        signingConfig signingConfigs.debug

        minifyEnabled true
        useProguard true

        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}

并遵循proguard规则访问app/proguard-rules.pro:

-keep class com.example.**{ *; }

# Flutter Wrapper

-keep class io.flutter.app.**{ *; }
-keep class io.flutter.plugin.**{ *; }
-keep class io.flutter.util.**{ *; }
-keep class io.flutter.view.**{ *; }
-keep class io.flutter.**{ *; }
-keep class io.flutter.plugins.**{ *; }

## ---------------Begin: proguard configuration for Gson  ----------

# Gson uses generic type information stored in a class file when working with fields. Proguard

# removes such information by default, so configure it to keep all of it.

-keepattributes Signature

# For using GSON @Expose annotation

-keepattributes *Annotation*

# Gson specific classes

-dontwarn sun.misc.**

# -keep class com.google.gson.stream.**{ *; }

# Application classes that will be serialized/deserialized over Gson

-keep class com.google.gson.examples.android.model.**{ <fields>; }

# Prevent proguard from stripping interface information from TypeAdapter, TypeAdapterFactory,

# JsonSerializer, JsonDeserializer instances (so they can be used in @JsonAdapter)

-keep class * extends com.google.gson.TypeAdapter
-keep class * implements com.google.gson.TypeAdapterFactory
-keep class * implements com.google.gson.JsonSerializer
-keep class * implements com.google.gson.JsonDeserializer

# Prevent R8 from leaving Data object members always null

-keepclassmembers,allowobfuscation class * {
  @com.google.gson.annotations.SerializedName <fields>;
}

# Retain generic signatures of TypeToken and its subclasses with R8 version 3.0 and higher.

-keep,allowobfuscation,allowshrinking class com.google.gson.reflect.TypeToken
-keep,allowobfuscation,allowshrinking class * extends com.google.gson.reflect.TypeToken

## ---------------End: proguard configuration for Gson  ----------