Implement checked mode for const constructors.

lib/compiler/implementation/constant_system_dart.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.
 
 const DART_CONSTANT_SYSTEM = const DartConstantSystem();
 
 class BitNotOperation implements UnaryOperation {
   final SourceString name = const SourceString('~');
   bool isUserDefinable() => true;
   const BitNotOperation();
@@ skipping 324 matching lines @@
   StringConstant createString(DartString string, Node diagnosticNode)
       => new StringConstant(string, diagnosticNode);
   BoolConstant createBool(bool value) => new BoolConstant(value);
   NullConstant createNull() => new NullConstant();
 
   bool isInt(Constant constant) => constant.isInt();
   bool isDouble(Constant constant) => constant.isDouble();
   bool isString(Constant constant) => constant.isString();
   bool isBool(Constant constant) => constant.isBool();
   bool isNull(Constant constant) => constant.isNull();
-}
+
+  bool isSubtype(Compiler compiler, DartType s, DartType t) {
+    return compiler.types.isSubtype(s, t);
+  }
+}