Added TODO.

lib/compiler/implementation/constants.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.
 
 abstract class ConstantVisitor<R> {
   R visitSentinel(SentinelConstant constant);
   R visitFunction(FunctionConstant constant);
   R visitNull(NullConstant constant);
   R visitInt(IntConstant constant);
   R visitDouble(DoubleConstant constant);
   R visitTrue(TrueConstant constant);
   R visitFalse(FalseConstant constant);
   R visitString(StringConstant constant);
   R visitList(ListConstant constant);
   R visitMap(MapConstant constant);
   R visitConstructed(ConstructedConstant constant);
 }
 
-class Constant {
+abstract class Constant {
   const Constant();
 
   bool isNull() => false;
   bool isBool() => false;
   bool isTrue() => false;
   bool isFalse() => false;
   bool isInt() => false;
   bool isDouble() => false;
   bool isNum() => false;
   bool isString() => false;
@@ skipping 51 matching lines @@
 
   DartType computeType(Compiler compiler) {
     return compiler.functionClass.computeType(compiler);
   }
 
   int hashCode() => (17 * element.hashCode()) & 0x7fffffff;
 
   accept(ConstantVisitor visitor) => visitor.visitFunction(this);
 }
 
-class PrimitiveConstant extends Constant {
+abstract class PrimitiveConstant extends Constant {
   abstract get value;
   const PrimitiveConstant();
   bool isPrimitive() => true;
 
   bool operator ==(var other) {
     if (other is !PrimitiveConstant) return false;
     PrimitiveConstant otherPrimitive = other;
     // We use == instead of === so that DartStrings compare correctly.
     return value == otherPrimitive.value;
   }
@@ skipping 21 matching lines @@
     buffer.add(JsNull);
   }
 
   // The magic constant has no meaning. It is just a random value.
   int hashCode() => 785965825;
   DartString toDartString() => const LiteralDartString("null");
 
   accept(ConstantVisitor visitor) => visitor.visitNull(this);
 }
 
-class NumConstant extends PrimitiveConstant {
+abstract class NumConstant extends PrimitiveConstant {
   abstract num get value;
   const NumConstant();
   bool isNum() => true;
 }
 
 class IntConstant extends NumConstant {
   final int value;
   factory IntConstant(int value) {
     switch (value) {
       case 0: return const IntConstant._internal(0);
@@ skipping 74 matching lines @@
       return value == otherValue;
     }
   }
 
   int hashCode() => value.hashCode();
   DartString toDartString() => new DartString.literal(value.toString());
 
   accept(ConstantVisitor visitor) => visitor.visitDouble(this);
 }
 
-class BoolConstant extends PrimitiveConstant {
+abstract class BoolConstant extends PrimitiveConstant {
   factory BoolConstant(value) {
     return value ? new TrueConstant() : new FalseConstant();
   }
   const BoolConstant._internal();
   bool isBool() => true;
 
   DartType computeType(Compiler compiler) {
     return compiler.boolClass.computeType(compiler);
   }
 
@@ skipping 59 matching lines @@
     return (_hashCode == otherString._hashCode) && (value == otherString.value);
   }
 
   int hashCode() => _hashCode;
   DartString toDartString() => value;
   int get length => value.length;
 
   accept(ConstantVisitor visitor) => visitor.visitString(this);
 }
 
-class ObjectConstant extends Constant {
+abstract class ObjectConstant extends Constant {
   final DartType type;
 
   ObjectConstant(this.type);
   bool isObject() => true;
 
   DartType computeType(Compiler compiler) => type;
 
   // TODO(1603): The class should be marked as abstract, but the VM doesn't
   // currently allow this.
   abstract int hashCode();
@@ skipping 115 matching lines @@
       if (fields[i] != other.fields[i]) return false;
     }
     return true;
   }
 
   int hashCode() => _hashCode;
   List<Constant> getDependencies() => fields;
 
   accept(ConstantVisitor visitor) => visitor.visitConstructed(this);
 }