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

Index: sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.dart	(revision 22108)
+++ sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.dart	(working copy)
@@ -231,9 +231,7 @@
 
   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);
@@ -302,7 +300,9 @@
     // 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.