anchor.diff:    bool isMutableArray() => true;
     String toString() => "mutable array";
   
  -  HType union(HType other, Compiler compiler) {
  +  HType unionOld(HType other, Compiler compiler) {
       if (other.isConflicting()) return HType.MUTABLE_ARRAY;
       if (other.isUnknown()) return HType.UNKNOWN;
       if (other.isMutableArray()) return HType.MUTABLE_ARRAY;
  
positive.code:     return HType.UNKNOWN;
    }
  
    HType intersection(HType other, Compiler compiler) {
      if (other.isConflicting()) return HType.CONFLICTING;
      if (other.isUnknown()) return HType.EXTENDABLE_ARRAY;
      if (other.isExtendableArray()) return HType.EXTENDABLE_ARRAY;
  
negative.code:     return HType.UNKNOWN;
    }
  
    HType intersectionOld(HType other, Compiler compiler) {
      if (other.isConflicting()) return HType.CONFLICTING;
      if (other.isUnknown()) return HType.READABLE_ARRAY;
      if (other.isString()) return HType.CONFLICTING;
  