Use platform_env instead of dart_env in angular tests.

The VM is about to switch to use the CFE's constant evaluator. This change
will break const.fromEnvironment on modular builds (which affects blaze and build_runner).

Because the only purpose of these definitions was to pass arguments to tests, we can
replace them with a runtime Platform.environment variable.

Context: b/129881700 and dart-lang/sdk#36513
PiperOrigin-RevId: 242916194

Author: Googler

_goldens/test/check_goldens_test.dart

@@ -17,7 +17,7 @@ void main() {
   if (_isBazel) {
     path = p.join(
       p.current,
-      const String.fromEnvironment('BAZEL_ROOT_PATH'),
+      Platform.environment['BAZEL_ROOT_PATH'],
     );
     if (path == null) {
       // Bazel specific: Use an environment variable to define the root path.

_tests/lib/compiler.dart

@@ -29,7 +29,7 @@ final Future<PackageAssetReader> _packageAssets = (() async {
   if (runfiles == null) {
     return PackageAssetReader.currentIsolate();
   }
-  final root = const String.fromEnvironment('PKG_ANGULAR_ROOT');
+  final root = Platform.environment['PKG_ANGULAR_ROOT'];
   final path = '$runfiles/$root';
   if (!FileSystemEntity.isFileSync('$path/lib/angular.dart')) {
     throw StateError('Could not find $path/lib/angular.dart');