code: function attributeToPropertyName(attributeName) {
    let propertyName = attributeToPropertyNames[attributeName];
    if (!propertyName) {
      // Convert and memoize.
      const hyphenRegEx = /-([a-z])/g;
      propertyName = attributeName.replace(hyphenRegEx,
          match => match[1].toUpperCase());
      attributeToPropertyNames[attributeName] = propertyName;
    }
    return propertyName;
  }
docs: Convert hyphenated foo-bar attribute name to camel case fooBar property name.