Move diagnostic_listener.dart and messages.dart to the diagnostics folder

tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart

 // Copyright (c) 2014, 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.
 
 // SExpressionUnstringifier implements the inverse operation to
 // [SExpressionStringifier].
 
 library sexpr_unstringifier;
 
 import 'package:compiler/src/constants/expressions.dart';
 import 'package:compiler/src/constants/values.dart';
 import 'package:compiler/src/dart_types.dart' as dart_types
     show DartType;
+import 'package:compiler/src/diagnostics/messages.dart'
+    show MessageKind;
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/modelx.dart'
     show ErroneousElementX, TypeVariableElementX;
-import 'package:compiler/src/messages.dart'
-    show MessageKind;
 import 'package:compiler/src/tree/tree.dart' show LiteralDartString;
 import 'package:compiler/src/universe/universe.dart'
     show Selector, SelectorKind, CallStructure;
 import 'package:compiler/src/cps_ir/cps_ir_nodes.dart';
 
 /// Used whenever a node constructed by [SExpressionUnstringifier] needs a
 /// named entity.
 class DummyEntity extends Entity {
   final String name;
   DummyEntity(this.name);
@@ skipping 845 matching lines @@
     tokens.consumeStart(SET_FIELD);
 
     Primitive object = name2variable[tokens.read()];
     Element field = new DummyElement(tokens.read());
     Primitive value = name2variable[tokens.read()];
 
     tokens.consumeEnd();
     return new SetField(object, field, value);
   }
 }