anchor.diff:    // TODO(9586): Move type info for static functions onto an interceptor.
     var interceptor = getInterceptor(object);
     var isSubclass = getField(interceptor, isField);
  -  // When we read the field and it is not there, [isSubclass] will be [:null:].
  -  if (isNull(isSubclass)) return false;
  +  // When we read the field and it is not there, [isSubclass] will be `null`.
  +  if (isSubclass == null) return false;
     // Should the asField function be passed the receiver?
     var substitution = getField(interceptor, asField);
     return checkArguments(substitution, arguments, checks);
  
positive.code: /**
   * Sets the runtime type information on [target]. [typeInfo] is a type
   * representation of type 4 or 5, that is, either a JavaScript array or
   * [:null:].
   */
  Object setRuntimeTypeInfo(Object target, var typeInfo) {
    assert(isNull(typeInfo) || isJsArray(typeInfo));
    // We have to check for null because factories may return null.
    if (target != null) JS('var', r'#.$builtinTypeInfo = #', target, typeInfo);
    return target;
  
negative.code: 
  hasNoField(var object, var name) => !hasField(object, name);
  
  /// Returns `true` if [o] is a JavaScript function.
  bool isJsFunction(var o) => JS('bool', r'typeof # == "function"', o);
  
  /// Returns `true` if [o] is a JavaScript object.
  bool isJsObject(var o) => JS('bool', r"typeof # == 'object'", o);
  
  /**
   * Returns `true` if the JavaScript values [s] and [t] are identical. We use
   * this helper instead of [identical] because `identical` needs to merge
   * `null` and `undefined` (which we can avoid).
   */
  bool isIdentical(var s, var t) => JS('bool', '# === #', s, t);
  
  /**
   * Returns `true` if the JavaScript values [s] and [t] are not identical. We use
   * this helper instead of [identical] because `identical` needs to merge
   * `null` and `undefined` (which we can avoid).
   */
  bool isNotIdentical(var s, var t) => JS('bool', '# !== #', s, t);
  