If an instruction is known to already by a number, the integer check will try to make it an integer.

lib/compiler/implementation/ssa/bailout.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.
 
 class BailoutInfo {
   int instructionId;
   int bailoutId;
   BailoutInfo(this.instructionId, this.bailoutId);
 }
 
@@ skipping 144 matching lines @@
     // generation of a bailout method.
     if (instruction is HIndex &&
         (instruction as HIndex).isBuiltin(types) &&
         hasTypeGuards) {
       HBasicBlock loopHeader = instruction.block.enclosingLoopHeader;
       if (loopHeader != null && loopHeader.parentLoopHeader != null) {
         return true;
       }
     }
 
+    // If the instruction is used by a phi where a guard would be
+    // valuable, put the guard on that instruction.
+    for (HInstruction user in instruction.usedBy) {
+      if (user is HPhi
+          && user.block.id > instruction.id
+          && typeGuardWouldBeValuable(user, speculativeType)) {
+        return true;
+      }
+    }
+
     // Insert type guards if the method is likely to be called in a
     // loop.
     return calledInLoop;
   }
 
   bool shouldInsertTypeGuard(HInstruction instruction,
                              HType speculativeType,
                              HType computedType) {
     if (!speculativeType.isUseful()) return false;
     // If the types agree we don't need to check.
@@ skipping 352 matching lines @@
         hasComplexBailoutTargets = true;
       }
     } else {
       hasComplexBailoutTargets = true;
       blocks.forEach((HBasicBlock block) {
         block.bailoutTargets.add(target);
       });
     }
   }
 }