We must also check if the first input of a phi is unknown.

lib/compiler/implementation/ssa/nodes.dart

 // Copyright (c) 2011, 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 visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
   R visitBitXor(HBitXor node);
@@ skipping 1824 matching lines @@
     assert(isInBasicBlock());
     inputs.add(input);
     input.usedBy.add(this);
   }
 
   // Compute the (shared) type of the inputs if any. If all inputs
   // have the same known type return it. If any two inputs have
   // different known types, we'll return a conflict -- otherwise we'll
   // simply return an unknown type.
   HType computeInputsType(bool unknownWins) {
-    bool seenUnknown = false;
     HType candidateType = inputs[0].propagatedType;
+    bool seenUnknown = candidateType.isUnknown();
     for (int i = 1, length = inputs.length; i < length; i++) {
       HType inputType = inputs[i].propagatedType;
       if (inputType.isUnknown()) {
         seenUnknown = true;
       } else {
         candidateType = candidateType.combine(inputType);
         if (candidateType.isConflicting()) return HType.CONFLICTING;
       }
     }
     if (seenUnknown && unknownWins) return HType.UNKNOWN;
@@ skipping 447 matching lines @@
   final bool isAnd;
   final SubExpression left;
   final SubExpression right;
   final HBasicBlock joinBlock;
   HAndOrBlockInformation(this.isAnd,
                          this.left,
                          this.right,
                          this.joinBlock);
   bool accept(HBlockInformationVisitor visitor) => visitor.visitAndOrInfo(this);
 }