Added TODO.

lib/compiler/implementation/ssa/nodes.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 HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
@@ skipping 50 matching lines @@
   R visitSubtract(HSubtract node);
   R visitSwitch(HSwitch node);
   R visitThis(HThis node);
   R visitThrow(HThrow node);
   R visitTruncatingDivide(HTruncatingDivide node);
   R visitTry(HTry node);
   R visitTypeGuard(HTypeGuard node);
   R visitTypeConversion(HTypeConversion node);
 }
 
-class HGraphVisitor {
+abstract class HGraphVisitor {
   visitDominatorTree(HGraph graph) {
     void visitBasicBlockAndSuccessors(HBasicBlock block) {
       visitBasicBlock(block);
       List dominated = block.dominatedBlocks;
       for (int i = 0; i < dominated.length; i++) {
         visitBasicBlockAndSuccessors(dominated[i]);
       }
     }
 
     visitBasicBlockAndSuccessors(graph.entry);
   }
 
   visitPostDominatorTree(HGraph graph) {
     void visitBasicBlockAndSuccessors(HBasicBlock block) {
       List dominated = block.dominatedBlocks;
       for (int i = dominated.length - 1; i >= 0; i--) {
         visitBasicBlockAndSuccessors(dominated[i]);
       }
       visitBasicBlock(block);
     }
 
     visitBasicBlockAndSuccessors(graph.entry);
   }
 
   abstract visitBasicBlock(HBasicBlock block);
 }
 
-class HInstructionVisitor extends HGraphVisitor {
+abstract class HInstructionVisitor extends HGraphVisitor {
   HBasicBlock currentBlock;
 
   abstract visitInstruction(HInstruction node);
 
   visitBasicBlock(HBasicBlock node) {
     void visitInstructionList(HInstructionList list) {
       HInstruction instruction = list.first;
       while (instruction !== null) {
         visitInstruction(instruction);
         instruction = instruction.next;
@@ skipping 2807 matching lines @@
   HBasicBlock get start => expression.start;
   HBasicBlock get end {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty());
     return statements.last().end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }