Make invocation namedArguments unmodifiable.

runtime/lib/invocation_mirror_patch.dart

 // Copyright (c) 2012, 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.
 
 class _InvocationMirror implements Invocation {
   // Constants describing the invocation type.
   // _FIELD cannot be generated by regular invocation mirrors.
   static const int _METHOD = 0;
   static const int _GETTER = 1;
   static const int _SETTER = 2;
@@ skipping 71 matching lines @@
       if (numNamedArguments == 0) {
         return _namedArguments = const <Symbol, dynamic>{};
       }
       _namedArguments = new Map<Symbol, dynamic>();
       for (int i = 0; i < numNamedArguments; i++) {
         String arg_name = _argumentsDescriptor[2 + 2*i];
         var arg_value = _arguments[_argumentsDescriptor[3 + 2*i]];
         _namedArguments[new internal.Symbol.unvalidated(arg_name)] =
             arg_value;
       }
+      _namedArguments = new Map.unmodifiable(_namedArguments);
     }
     return _namedArguments;
   }
 
   bool get isMethod {
     if (_type == null) {
       _setMemberNameAndType();
     }
     return (_type & _TYPE_MASK) == _METHOD;
   }
@@ skipping 27 matching lines @@
   static _allocateInvocationMirror(String functionName,
                                    List argumentsDescriptor,
                                    List arguments,
                                    bool isSuperInvocation) {
     return new _InvocationMirror(functionName,
                                  argumentsDescriptor,
                                  arguments,
                                  isSuperInvocation);
   }
 }