Added TODO.

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 35 matching lines @@
       return DART_CONSTANT_SYSTEM.createBool(!boolConstant.value);
     }
     return null;
   }
   apply(value) => !value;
 }
 
 /**
  * Operations that only work if both arguments are integers.
  */
-class BinaryBitOperation implements BinaryOperation {
+abstract class BinaryBitOperation implements BinaryOperation {
   bool isUserDefinable() => true;
   const BinaryBitOperation();
   Constant fold(Constant left, Constant right) {
     if (left.isInt() && right.isInt()) {
       IntConstant leftInt = left;
       IntConstant rightInt = right;
       int resultValue = foldInts(leftInt.value, rightInt.value);
       if (resultValue === null) return null;
       return DART_CONSTANT_SYSTEM.createInt(resultValue);
     }
@@ skipping 39 matching lines @@
 class ShiftRightOperation extends BinaryBitOperation {
   final SourceString name = const SourceString('>>');
   const ShiftRightOperation();
   int foldInts(int left, int right) {
     if (right < 0) return null;
     return left >> right;
   }
   apply(left, right) => left >> right;
 }
 
-class BinaryBoolOperation implements BinaryOperation {
+abstract class BinaryBoolOperation implements BinaryOperation {
   bool isUserDefinable() => false;
   const BinaryBoolOperation();
   Constant fold(Constant left, Constant right) {
     if (left.isBool() && right.isBool()) {
       BoolConstant leftBool = left;
       BoolConstant rightBool = right;
       bool resultValue = foldBools(leftBool.value, rightBool.value);
       return DART_CONSTANT_SYSTEM.createBool(resultValue);
     }
     return null;
   }
 
   abstract bool foldBools(bool left, bool right);
 }
 
 class BooleanAndOperation extends BinaryBoolOperation {
   final SourceString name = const SourceString('&&');
   const BooleanAndOperation();
   bool foldBools(bool left, bool right) => left && right;
   apply(left, right) => left && right;
 }
 
 class BooleanOrOperation extends BinaryBoolOperation {
   final SourceString name = const SourceString('||');
   const BooleanOrOperation();
   bool foldBools(bool left, bool right) => left || right;
   apply(left, right) => left || right;
 }
 
-class ArithmeticNumOperation implements BinaryOperation {
+abstract class ArithmeticNumOperation implements BinaryOperation {
   bool isUserDefinable() => true;
   const ArithmeticNumOperation();
   Constant fold(Constant left, Constant right) {
     if (left.isNum() && right.isNum()) {
       NumConstant leftNum = left;
       NumConstant rightNum = right;
       num foldedValue;
       if (left.isInt() && right.isInt()) {
         foldedValue = foldInts(leftNum.value, rightNum.value);
       } else {
@@ skipping 75 matching lines @@
       NumConstant rightNum = right;
       double result = leftNum.value + rightNum.value;
       return DART_CONSTANT_SYSTEM.createDouble(result);
     } else {
       return null;
     }
   }
   apply(left, right) => left + right;
 }
 
-class RelationalNumOperation implements BinaryOperation {
+abstract class RelationalNumOperation implements BinaryOperation {
   bool isUserDefinable() => true;
   const RelationalNumOperation();
   Constant fold(Constant left, Constant right) {
     if (left.isNum() && right.isNum()) {
       NumConstant leftNum = left;
       NumConstant rightNum = right;
       bool foldedValue = foldNums(leftNum.value, rightNum.value);
       assert(foldedValue != null);
       return DART_CONSTANT_SYSTEM.createBool(foldedValue);
     }
@@ skipping 109 matching lines @@
   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);
   }
 }