Relax string literal requirements on arguments

pkg/intl/lib/src/intl_message.dart

Index: pkg/intl/lib/src/intl_message.dart
diff --git a/pkg/intl/lib/src/intl_message.dart b/pkg/intl/lib/src/intl_message.dart
index b21c0b85812c0ee5472a83c126334befd61e2ca0..b5bc0fed3c7875242b97be8d9eda5f5da349bff3 100644
--- a/pkg/intl/lib/src/intl_message.dart
+++ b/pkg/intl/lib/src/intl_message.dart
@@ -99,9 +99,8 @@ abstract class Message {
         && ["desc", "name"].contains(each.name.label.name));
     var values = simpleArguments.map((each) => each.expression).toList();
     for (var arg in values) {
-      if (arg is! SimpleStringLiteral) {
-        return "Intl.message argument '$arg' must be "
-            "a simple string literal";
+      if (arg is! StringLiteral) {
+        return( "Intl.message arguments must be string literals: $arg");
       }
     }
     return null;