Address review comments from https://chromiumcodereview.appspot.com/10454049/.

lib/compiler/implementation/ssa/validate.dart

 // Copyright (c) 2011, 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.
 
 class HValidator extends HInstructionVisitor {
   bool isValid = true;
   HGraph graph;
 
   void visitGraph(HGraph visitee) {
     graph = visitee;
@@ skipping 38 matching lines @@
     if (block.last is HThrow && !block.successors.isEmpty()) {
       markInvalid("Throw block with successor");
     }
 
     if (block.successors.isEmpty() &&
         block.last is !HThrow &&
         !block.isExitBlock()) {
       markInvalid("Non-exit or throw block without successor");
     }
 
-    // Make sure that successors ids are always higher than the current one.
+    // Check that successors ids are always higher than the current one.
     // TODO(floitsch): this is, of course, not true for back-branches.
     if (block.id === null) markInvalid("block without id");
     for (HBasicBlock successor in block.successors) {
       if (!isValid) break;
       if (successor.id === null) markInvalid("successor without id");
       if (successor.id <= block.id && !successor.isLoopHeader()) {
         markInvalid("successor with lower id, but not a loop-header");
       }
     }
 
-    // Make sure that the entries in the dominated-list are sorted.
+    // Check that the entries in the dominated-list are sorted.
     int lastId = 0;
     for (HBasicBlock dominated in block.dominatedBlocks) {
       if (!isValid) break;
       if (dominated.dominator !== block) {
         markInvalid("dominated block not pointing back");
       }
       if (dominated.id === null || dominated.id <= lastId) {
         markInvalid("dominated.id === null or dominated has <= id");
       }
       lastId = dominated.id;
     }
 
     if (!isValid) return;
     block.forEachPhi(visitInstruction);
 
-    // Make sure the parameters of a phi are dominating the
-    // corresponding predecessor block.
+    // Check that the blocks of the parameters of a phi are dominating the
+    // corresponding predecessor block. Note that a block dominates
+    // itself.
     block.forEachPhi((HPhi phi) {
       for (int i = 0; i < phi.inputs.length; i++) {
         HInstruction input = phi.inputs[i];
         if (!input.block.dominates(block.predecessors[i])) {
           markInvalid("Definition does not dominate use");
         }
       }
     });
 
-    // Make sure the inputs of an instruction dominate the
-    // instruction.
+    // Check that the blocks of the inputs of an instruction dominate the
+    // instruction's block.
     block.forEachInstruction((HInstruction instruction) {
       for (HInstruction input in instruction.inputs) {
         if (!input.block.dominates(block)) {
           markInvalid("Definition does not dominate use");
         }
       }
     });
 
     super.visitBasicBlock(block);
   }
@@ skipping 57 matching lines @@
       markInvalid("Instruction in wrong block");
     }
     if (!hasCorrectInputs()) {
       markInvalid("Incorrect inputs");
     }
     if (!hasCorrectUses()) {
       markInvalid("Incorrect uses");
     }
   }
 }