function mixSpecIntoComponent(Constructor, spec) {
  ("production" !== "development" ? invariant(
    !ReactDescriptor.isValidFactory(spec),
    'ReactCompositeComponent: You\'re attempting to ' +
    'use a component class as a mixin. Instead, just use a regular object.'
  ) : invariant(!ReactDescriptor.isValidFactory(spec)));
  ("production" !== "development" ? invariant(
    !ReactDescriptor.isValidDescriptor(spec),
    'ReactCompositeComponent: You\'re attempting to ' +
    'use a component as a mixin. Instead, just use a regular object.'
  ) : invariant(!ReactDescriptor.isValidDescriptor(spec)));

  var proto = Constructor.prototype;
  for (var name in spec) {
    var property = spec[name];
    if (!spec.hasOwnProperty(name)) {
      continue;
    }

    validateMethodOverride(proto, name);

    if (RESERVED_SPEC_KEYS.hasOwnProperty(name)) {
      RESERVED_SPEC_KEYS[name](Constructor, property);
    } else {
      // Setup methods on prototype:
      // The following member methods should not be automatically bound:
      // 1. Expected ReactCompositeComponent methods (in the "interface").
      // 2. Overridden methods (that were mixed in).
      var isCompositeComponentMethod =
        ReactCompositeComponentInterface.hasOwnProperty(name);
      var isAlreadyDefined = proto.hasOwnProperty(name);
      var markedDontBind = property && property.__reactDontBind;
      var isFunction = typeof property === 'function';
      var shouldAutoBind =
        isFunction &&
        !isCompositeComponentMethod &&
        !isAlreadyDefined &&
        !markedDontBind;

      if (shouldAutoBind) {
        if (!proto.__reactAutoBindMap) {
          proto.__reactAutoBindMap = {};
        }
        proto.__reactAutoBindMap[name] = property;
        proto[name] = property;
      } else {
        if (isAlreadyDefined) {
          var specPolicy = ReactCompositeComponentInterface[name];

          // These cases should already be caught by validateMethodOverride
          ("production" !== "development" ? invariant(
            isCompositeComponentMethod && (
              specPolicy === SpecPolicy.DEFINE_MANY_MERGED ||
              specPolicy === SpecPolicy.DEFINE_MANY
            ),
            'ReactCompositeComponent: Unexpected spec policy %s for key %s ' +
            'when mixing in component specs.',
            specPolicy,
            name
          ) : invariant(isCompositeComponentMethod && (
            specPolicy === SpecPolicy.DEFINE_MANY_MERGED ||
            specPolicy === SpecPolicy.DEFINE_MANY
          )));

          // For methods which are defined more than once, call the existing
          // methods before calling the new property, merging if appropriate.
          if (specPolicy === SpecPolicy.DEFINE_MANY_MERGED) {
            proto[name] = createMergedResultFunction(proto[name], property);
          } else if (specPolicy === SpecPolicy.DEFINE_MANY) {
            proto[name] = createChainedFunction(proto[name], property);
          }
        } else {
          proto[name] = property;
          if ("production" !== "development") {
            // Add verbose displayName to the function, which helps when looking
            // at profiling tools.
            if (typeof property === 'function' && spec.displayName) {
              proto[name].displayName = spec.displayName + '_' + name;
            }
          }
        }
      }
    }
  }
}
Custom version of `mixInto` which handles policy validation and reserved
specification keys when building `ReactCompositeComponent` classses.