Remove some warnings and dead code in dart2js.

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 113 matching lines @@
     // values. Furthermore NaN !== NaN.
     if (left.isNum() && right.isNum()) {
       NumConstant leftNum = left;
       NumConstant rightNum = right;
       double leftDouble = leftNum.value.toDouble();
       double rightDouble = rightNum.value.toDouble();
       return new BoolConstant(leftDouble == rightDouble);
     }
     return result;
   }
+
+  apply(left, right) => identical(left, right);
 }
 
 /**
  * Constant system following the semantics for Dart code that has been
  * compiled to JavaScript.
  */
 class JavaScriptConstantSystem implements ConstantSystem {
   const int BITS31 = 0x8FFFFFFF;
   const int BITS32 = 0xFFFFFFFF;
   // The maximum integer value a double can represent without losing
@@ skipping 83 matching lines @@
     // 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);
   }
 }