Relax string literal requirements on arguments

pkg/intl/lib/src/intl_message.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * This provides classes to represent the internal structure of the
  * arguments to `Intl.message`. It is used when parsing sources to extract
  * messages or to generate code for message substitution. Normal programs
  * using Intl would not import this library.
  *
@@ skipping 81 matching lines @@
     if (outerName != null && outerName != messageName.expression.value) {
       return "The 'name' argument for Intl.message must match "
           "the name of the containing function ("
           "'${messageName.expression.value}' vs. '$outerName')";
     }
     var simpleArguments = arguments.where(
         (each) => each is NamedExpression
         && ["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;
   }
 
   /**
    * Turn a value, typically read from a translation file or created out of an
    * AST for a source program, into the appropriate
    * subclass. We expect to get literal Strings, variable substitutions
    * represented by integers, things that are already MessageChunks and
@@ skipping 560 matching lines @@
     out.write('(');
     out.write(mainArgument);
     var args = codeAttributeNames;
     out.write(", {");
     args.fold(out, (buffer, arg) => buffer..write(
         "'$arg': '${this[arg].toCode()}', "));
     out.write("})}");
     return out.toString();
   }
 }