Allow creating VariableSubstitution by name or by index

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 215 matching lines @@
 class LiteralString extends Message {
   String string;
   LiteralString(this.string, Message parent) : super(parent);
   toCode() => escapeAndValidateString(string);
   toString() => "Literal($string)";
   String expanded([Function f = _nullTransform]) => f(this, string);
 }
 
 /**
  * Represents an interpolation of a variable value in a message. We expect
- * this to be specified as an [index] into the list of variables, and we will
- * compute the variable name for the interpolation based on that.
+ * this to be specified as an [index] into the list of variables, or else
+ * as the name of a variable that exists in [arguments] and we will
+ * compute the variable name or the index based on the value of the other.
  */
 class VariableSubstitution extends Message {
-  VariableSubstitution(this.index, Message parent) : super(parent);
+  VariableSubstitution(this._index, Message parent) : super(parent);
+  VariableSubstitution.named(this._variableName, Message parent)
+      : super(parent);
 
   /** The index in the list of parameters of the containing function. */
-  int index;
+  int _index;
+  int get index {
+    if (_index != null) return _index;
+    if (arguments.isEmpty) return null;
+    return _index = arguments.indexOf(_variableName);
+  }
 
   /**
    * The name of the variable in the parameter list of the containing function.
    * Used when generating code for the interpolation.
    */
   String get variableName =>
       _variableName == null ? _variableName = arguments[index] : _variableName;
   String _variableName;
   // Although we only allow simple variable references, we always enclose them
   // in curly braces so that there's no possibility of ambiguity with
@@ skipping 380 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();
   }
 }