Enable Proguard by default on release mode (#39986)

diff --git a/packages/flutter_tools/lib/src/build_info.dart b/packages/flutter_tools/lib/src/build_info.dart
index 5331465..cb7afb8 100644
--- a/packages/flutter_tools/lib/src/build_info.dart
+++ b/packages/flutter_tools/lib/src/build_info.dart
@@ -92,6 +92,7 @@
       AndroidArch.arm64_v8a,
     ],
     this.splitPerAbi = false,
+    this.proguard = false,
   });
 
   // The build info containing the mode and flavor.
@@ -104,6 +105,9 @@
   /// will be produced.
   final bool splitPerAbi;
 
+  /// Whether to enable Proguard on release mode.
+  final bool proguard;
+
   /// The target platforms for the build.
   final Iterable<AndroidArch> targetArchs;
 }