Add an optional parameter to the dominatedUsers function in case we want to analyze the whole block,

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.
 
 interface HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
@@ skipping 914 matching lines @@
       if (oldInputUsers[i] == this) {
         oldInputUsers[i] = oldInputUsers[oldInput.usedBy.length - 1];
         oldInputUsers.length--;
       } else {
         i++;
       }
     }
   }
 
   // Compute the set of users of this instruction that is dominated by
-  // [other].
-  Set<HInstruction> dominatedUsers(HInstruction other) {
+  // [other] or [otherBlock].
+  Set<HInstruction> dominatedUsers(HInstruction other,

[kasperl, 2012/08/20 11:37:22]

I'd prefer two different entry points for this: 

   1. usersDominatedByInstruction(HInstruction)
   2. usersDominatedByBlock(HBlock)

The current method might be helper function for implementing the two functions
and you can assert that we're not passing in both an instruction and a block.

+                                   [HBasicBlock otherBlock = null]) {
     // Keep track of all instructions that we have to deal with later
     // and count the number of them that are in the current block.
     Set<HInstruction> users = new Set<HInstruction>();
     int usersInCurrentBlock = 0;
+    if (otherBlock == null) otherBlock = other.block;
 
     // Run through all the users and see if they are dominated or
-    // potentially dominated by [other].
-    HBasicBlock otherBlock = other.block;
+    // potentially dominated by [otherBlock].

[kasperl, 2012/08/20 11:37:22]

Here you're adding all the users that are dominated by the entry instruction in
the otherBlock, so you should probably update the comment to not contain "or
potentially dominated" because they all are dominated by otherBlock.

     for (int i = 0, length = usedBy.length; i < length; i++) {
       HInstruction current = usedBy[i];
       if (current !== other && otherBlock.dominates(current.block)) {
         if (current.block === otherBlock) usersInCurrentBlock++;
         users.add(current);
       }
     }
 
-    // Run through all the phis in the same block as [other] and remove them
+    // Run through all the phis in otherBlock and remove them
     // from the users set.
     if (usersInCurrentBlock > 0) {
       for (HPhi phi = otherBlock.phis.first; phi !== null; phi = phi.next) {
         if (users.contains(phi)) {
           users.remove(phi);
           if (--usersInCurrentBlock == 0) break;
         }
       }
     }
 
-    // Run through all the instructions before [other] and remove them
+    // Run through all the instructions before otherBlock and remove them

[kasperl, 2012/08/20 11:37:22]

This should still be all instructions before [other], right?

     // from the users set.
-    if (usersInCurrentBlock > 0) {
+    if (other != null && usersInCurrentBlock > 0) {
       HInstruction current = otherBlock.first;
       while (current !== other) {
         if (users.contains(current)) {
           users.remove(current);
           if (--usersInCurrentBlock == 0) break;
         }
         current = current.next;
       }
     }
 
@@ skipping 1765 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);
 }