When a phi is generated at use site, the names of its inputs must be freed by the user of the phi.

sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.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.
 
 part of ssa;
 
 /**
  * The [LiveRange] class covers a range where an instruction is live.
  */
 class LiveRange {
@@ skipping 213 matching lines @@
 
   HInstruction unwrap(instruction) {
     do {
       instruction = instruction.checkedInput;
     } while (instruction is HCheck);
     return instruction;
   }
 
   void markAsLiveInEnvironment(HInstruction instruction,
                                LiveEnvironment environment) {
-    // The inputs of a [HPhi] are being handled at the entry of a
-    // block.
-    if (generateAtUseSite.contains(instruction) && instruction is !HPhi) {
+    if (generateAtUseSite.contains(instruction)) {
       markInputsAsLiveInEnvironment(instruction, environment);
     } else {
       environment.add(instruction, instructionId);
       // Special case the HCheck instruction to mark the actual
       // checked instruction live. The checked instruction and the
       // [HCheck] will share the same live ranges.
       if (instruction is HCheck) {
         HInstruction checked = unwrap(instruction);
         if (!generateAtUseSite.contains(checked)) {
           environment.add(checked, instructionId);
@@ skipping 48 matching lines @@
         removeFromEnvironment(instruction, environment);
         markInputsAsLiveInEnvironment(instruction, environment);
       }
       instructionId--;
       instruction = instruction.previous;
     }
 
     // We just remove the phis from the environment. The inputs of the
     // phis will be put in the environment of the predecessors.
     for (HPhi phi = block.phis.first; phi != null; phi = phi.next) {
-      environment.remove(phi, instructionId);
+      if (!generateAtUseSite.contains(phi)) {
+        environment.remove(phi, instructionId);
+      }
     }
 
     // Save the liveInstructions of that block.
     environment.startId = instructionId + 1;
     liveInstructions[block] = environment;
 
     // If the block is a loop header, we can remove the loop marker,
     // because it will just recompute the loop phis.
     // We also check if this loop header has any back edges. If not,
     // we know there is no loop marker for it.
@@ skipping 376 matching lines @@
       if (!needsName(input)) {
         names.addAssignment(predecessor, input, phi);
       } else {
         names.addCopy(predecessor, input, phi);
       }
     }
 
     namer.allocateName(phi);
   }
 }