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

pkg/compiler/lib/src/compile_time_constants.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.
 
 library dart2js.compile_time_constant_evaluator;
 
 import 'common/tasks.dart' show
     CompilerTask;
 import 'compiler.dart' show
     Compiler;
 import 'constant_system_dart.dart';
 import 'constants/constant_system.dart';
 import 'constants/evaluation.dart';
 import 'constants/expressions.dart';
 import 'constants/values.dart';
 import 'dart_types.dart';
 import 'diagnostics/invariant.dart' show
     invariant;
+import 'diagnostics/messages.dart' show
+    MessageKind;
 import 'enqueue.dart' show
     WorldImpact;
 import 'elements/elements.dart';
-import 'elements/modelx.dart' show FunctionElementX;
-import 'messages.dart' show MessageKind;
+import 'elements/modelx.dart' show
+    FunctionElementX;
 import 'resolution/resolution.dart';
 import 'resolution/operators.dart';
 import 'tree/tree.dart';
-import 'util/util.dart' show Link;
-import 'universe/universe.dart' show CallStructure;
+import 'util/util.dart' show
+    Link;
+import 'universe/universe.dart' show
+    CallStructure;
 
 /// A [ConstantEnvironment] provides access for constants compiled for variable
 /// initializers.
 abstract class ConstantEnvironment {
   /// The [ConstantSystem] used by this environment.
   ConstantSystem get constantSystem;
 
   /// Returns the constant value computed for [expression].
   // TODO(johnniwinther): Support directly evaluation of [expression].
   ConstantValue getConstantValue(ConstantExpression expression);
@@ skipping 1222 matching lines @@
 class _CompilerEnvironment implements Environment {
   final Compiler compiler;
 
   _CompilerEnvironment(this.compiler);
 
   @override
   String readFromEnvironment(String name) {
     return compiler.fromEnvironment(name);
   }
 }