Treat DownCastComposite as a StaticWarning, not a StaticTypeWarning.

pkg/analyzer/lib/src/task/strong/info.dart

 // Copyright (c) 2015, 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.
 
 /// Defines static information collected by the type checker and used later by
 /// emitters to generate code.
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.info;
 
@@ skipping 152 matching lines @@
 /// A down cast to a non-ground type.  These behave differently from standard
 /// Dart and may be more likely to fail at runtime.
 class DownCastComposite extends DownCast {
   DownCastComposite(TypeSystem rules, Expression expression, DartType fromType,
       DartType toType)
       : super._internal(rules, expression, fromType, toType);
 
   @override
   String get name => 'STRONG_MODE_DOWN_CAST_COMPOSITE';
 
-  toErrorCode() => new StaticTypeWarningCode(name, message);
+  toErrorCode() => new StaticWarningCode(name, message);
 }
 
 /// A down cast to a non-ground type.  These behave differently from standard
 /// Dart and may be more likely to fail at runtime.
 class DownCastImplicit extends DownCast {
   DownCastImplicit(TypeSystem rules, Expression expression, DartType fromType,
       DartType toType)
       : super._internal(rules, expression, fromType, toType);
 
   @override
@@ skipping 355 matching lines @@
         super(expression);
 
   @override
   List<Object> get arguments => [node, baseType, expectedType];
   @override
   String get message => 'Type check failed: {0} ({1}) is not of type {2}';
 
   @override
   String get name => 'STRONG_MODE_STATIC_TYPE_ERROR';
 }