Mark defaultTargetPlatform as constant for non-debug non-web builds. (#141105)

This PR adds the Dart VM `vm:platform-const-if` pragma introduced in
https://github.com/dart-lang/sdk/commit/57a1168875 to the
`defaultTargetPlatform` property, allowing it to be computed as if it
was a constant field in non-debug AOT builds. In particular, this means
that platform-specific code executed conditionally based on this
property can be tree-shaken in release builds. Note that this PR changes
`defaultTargetPlatform` to only allow overriding via
`debugDefaultTargetPlatformOverride` in debug builds, and makes it so
that compilation throws an error if code assigns
to`debugDefaultTargetPlatformOverride` in other build modes.

Related issue: #14233

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] All existing and new tests are passing.

packages/flutter/lib/src/foundation/_platform_io.dart

@@ -4,11 +4,13 @@
 
 import 'dart:io';
 import 'assertions.dart';
+import 'constants.dart';
 import 'platform.dart' as platform;
 
 export 'platform.dart' show TargetPlatform;
 
 /// The dart:io implementation of [platform.defaultTargetPlatform].
+@pragma('vm:platform-const-if', !kDebugMode)
 platform.TargetPlatform get defaultTargetPlatform {
   platform.TargetPlatform? result;
   if (Platform.isAndroid) {
@@ -30,7 +32,7 @@ platform.TargetPlatform get defaultTargetPlatform {
     }
     return true;
   }());
-  if (platform.debugDefaultTargetPlatformOverride != null) {
+  if (kDebugMode && platform.debugDefaultTargetPlatformOverride != null) {
     result = platform.debugDefaultTargetPlatformOverride;
   }
   if (result == null) {

packages/flutter/lib/src/foundation/platform.dart

@@ -4,6 +4,8 @@
 
 import '_platform_io.dart'
   if (dart.library.js_util) '_platform_web.dart' as platform;
+import 'assertions.dart';
+import 'constants.dart';
 
 /// The [TargetPlatform] that matches the platform on which the framework is
 /// currently executing.
@@ -22,7 +24,7 @@ import '_platform_io.dart'
 /// originally written assuming Android-like behavior, and we added platform
 /// adaptations for iOS later). Tests can check iOS behavior by using the
 /// platform override APIs (such as [ThemeData.platform] in the material
-/// library) or by setting [debugDefaultTargetPlatformOverride].
+/// library) or by setting [debugDefaultTargetPlatformOverride] in debug builds.
 ///
 /// Tests can also create specific platform tests by and adding a `variant:`
 /// argument to the test and using a [TargetPlatformVariant].
@@ -42,6 +44,7 @@ import '_platform_io.dart'
 // that would mean we'd be stuck with that platform forever emulating the other,
 // and we'd never be able to introduce dedicated behavior for that platform
 // (since doing so would be a big breaking change).
+@pragma('vm:platform-const-if', !kDebugMode)
 TargetPlatform get defaultTargetPlatform => platform.defaultTargetPlatform;
 
 /// The platform that user interaction should adapt to target.
@@ -76,7 +79,7 @@ enum TargetPlatform {
   windows,
 }
 
-/// Override the [defaultTargetPlatform].
+/// Override the [defaultTargetPlatform] in debug builds.
 ///
 /// Setting this to null returns the [defaultTargetPlatform] to its original
 /// value (based on the actual current platform).
@@ -94,5 +97,16 @@ enum TargetPlatform {
 /// certainly widgets to work assuming the presence of a system-wide back
 /// button, which will make those widgets unusable since iOS has no such button.
 ///
-/// In general, therefore, this property should not be used in release builds.
-TargetPlatform? debugDefaultTargetPlatformOverride;
+/// Attempting to override this property in non-debug builds causes an error.
+TargetPlatform? get debugDefaultTargetPlatformOverride =>
+    _debugDefaultTargetPlatformOverride;
+
+set debugDefaultTargetPlatformOverride(TargetPlatform? value) {
+  if (!kDebugMode) {
+    throw FlutterError(
+      'Cannot modify debugDefaultTargetPlatformOverride in non-debug builds.');
+  }
+  _debugDefaultTargetPlatformOverride = value;
+}
+
+TargetPlatform? _debugDefaultTargetPlatformOverride;