We run phases in-between GVN and code motion, so we have to handle cases in the SsaCodeMotion phase…

sdk/lib/_internal/compiler/implementation/ssa/optimize.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/optimize.dart	(revision 30948)
+++ sdk/lib/_internal/compiler/implementation/ssa/optimize.dart	(working copy)
@@ -1448,9 +1448,13 @@
       }
       if (!canBeMoved) continue;
 
-      // This is safe because we are running after GVN.
-      // TODO(ngeoffray): ensure GVN has been run.
-      set_.add(current);
+      HInstruction existing = set_.lookup(current);
+      if (existing == null) {
+        set_.add(current);
+      } else {
+        block.rewriteWithBetterUser(current, existing);
+        block.remove(current);
+      }
     }
   }
 }