Use `--target-os` for appropriate precompiled targets. (#127567)

This PR adds uses of the `--target-os` command line argument when
building kernel sources for precompiled applications for supported
target operating systems. The Dart CFE then:

* treats `Platform.operatingSystem` as if it were defined as the
constant string provided as an argument to the flag,
* treats `Platform.pathSeparator` as the appropriate separator for that
operating system,
* attempts to constant evaluate the initializer for any field annotated
with the `vm:platform-const` pragma, and
* attempts to constant evaluate all calls to a method annotated with the
`vm:platform-const` pragma.

The `vm:platform-const` pragma can appear in either library or user
code. If the attempt to constant evaluate the field initializer or
method call fails, then an error is thrown at kernel compilation time.

Addresses #14233.

The tests in
`packages/flutter_tools/test/general.shard/build_system/targets/common_test.dart`
have been adjusted properly to account for the new passed command line
arguments.

packages/flutter_tools/lib/src/build_system/targets/common.dart

@@ -206,6 +206,21 @@ class KernelSnapshot extends Target {
         forceLinkPlatform = false;
     }
 
+    final String? targetOS = switch (targetPlatform) {
+      TargetPlatform.fuchsia_arm64 || TargetPlatform.fuchsia_x64 => 'fuchsia',
+      TargetPlatform.android ||
+      TargetPlatform.android_arm ||
+      TargetPlatform.android_arm64 ||
+      TargetPlatform.android_x64 ||
+      TargetPlatform.android_x86 =>
+        'android',
+      TargetPlatform.darwin => 'macos',
+      TargetPlatform.ios => 'ios',
+      TargetPlatform.linux_arm64 || TargetPlatform.linux_x64 => 'linux',
+      TargetPlatform.windows_x64 => 'windows',
+      TargetPlatform.tester || TargetPlatform.web_javascript => null,
+    };
+
     final PackageConfig packageConfig = await loadPackageConfigWithLogging(
       packagesFile,
       logger: environment.logger,
@@ -234,6 +249,7 @@ class KernelSnapshot extends Target {
       dartDefines: decodeDartDefines(environment.defines, kDartDefines),
       packageConfig: packageConfig,
       buildDir: environment.buildDir,
+      targetOS: targetOS,
       checkDartPluginRegistry: environment.generateDartPluginRegistry,
     );
     if (output == null || output.errorCount != 0) {

packages/flutter_tools/lib/src/compile.dart

@@ -233,6 +233,7 @@ class KernelCompiler {
     String? initializeFromDill,
     String? platformDill,
     Directory? buildDir,
+    String? targetOS,
     bool checkDartPluginRegistry = false,
     required String? packagesPath,
     required BuildMode buildMode,
@@ -293,6 +294,11 @@ class KernelCompiler {
       if (aot) ...<String>[
         '--aot',
         '--tfa',
+        // The --target-os flag only makes sense for whole program compilation.
+        if (targetOS != null) ...<String>[
+          '--target-os',
+          targetOS,
+        ],
       ],
       if (packagesPath != null) ...<String>[
         '--packages',

packages/flutter_tools/test/general.shard/build_system/targets/common_test.dart

@@ -94,6 +94,8 @@ void main() {
         '--track-widget-creation',
         '--aot',
         '--tfa',
+        '--target-os',
+        'android',
         '--packages',
         '/.dart_tool/package_config.json',
         '--output-dill',
@@ -132,6 +134,8 @@ void main() {
         '--track-widget-creation',
         '--aot',
         '--tfa',
+        '--target-os',
+        'android',
         '--packages',
         '/.dart_tool/package_config.json',
         '--output-dill',
@@ -171,6 +175,8 @@ void main() {
         '--track-widget-creation',
         '--aot',
         '--tfa',
+        '--target-os',
+        'android',
         '--packages',
         '/.dart_tool/package_config.json',
         '--output-dill',
@@ -211,6 +217,8 @@ void main() {
         '--track-widget-creation',
         '--aot',
         '--tfa',
+        '--target-os',
+        'android',
         '--packages',
         '/.dart_tool/package_config.json',
         '--output-dill',