Implement checked mode for const constructors.

lib/compiler/implementation/js_backend/constant_system_javascript.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 JAVA_SCRIPT_CONSTANT_SYSTEM = const JavaScriptConstantSystem();
 
 class JavaScriptBitNotOperation extends BitNotOperation {
   const JavaScriptBitNotOperation();
 
   Constant fold(Constant constant) {
@@ skipping 199 matching lines @@
   BoolConstant createBool(bool value) => new BoolConstant(value);
   NullConstant createNull() => new NullConstant();
 
   // Integer checks don't verify that the number is not -0.0.
   bool isInt(Constant constant) => constant.isInt() || constant.isMinusZero();
   bool isDouble(Constant constant)
       => constant.isDouble() && !constant.isMinusZero();
   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) {
+    // At runtime, an integer is both an integer and a double: the
+    // integer type check is Math.floor, which will return true only
+    // for real integers, and our double type check is 'typeof number'
+    // which will return true for both integers and doubles.
+    if (s.element == compiler.intClass && t.element == compiler.doubleClass) {
+      return true;
+    }
+    return compiler.types.isSubtype(s, t);
+  }
+}