Set a normalized attribute on the element in a way such that all directives
can share the attribute. This function properly handles boolean attributes.
@param {string} key Normalized key. (ie ngAttribute)
@param {string|boolean} value The value to set. If `null` attribute will be deleted.
@param {boolean=} writeAttr If false, does not write the value to DOM element attribute.
Defaults to true.
@param {string=} attrName Optional none normalized name. Defaults to key.
function(key, value, writeAttr, attrName) {
        // TODO: decide whether or not to throw an error if "class"
        // is set through this function since it may cause $updateClass to
        // become unstable.

        var node = this.$$element[0],
            booleanKey = getBooleanAttrName(node, key),
            aliasedKey = getAliasedAttrName(key),
            observer = key,
            nodeName;

        if (booleanKey) {
          this.$$element.prop(key, value);
          attrName = booleanKey;
        } else if (aliasedKey) {
          this[aliasedKey] = value;
          observer = aliasedKey;
        }

        this[key] = value;

        // translate normalized key to actual key
        if (attrName) {
          this.$attr[key] = attrName;
        } else {
          attrName = this.$attr[key];
          if (!attrName) {
            this.$attr[key] = attrName = snake_case(key, '-');
          }
        }

        nodeName = nodeName_(this.$$element);

        // Sanitize img[srcset] values.
        if (nodeName === 'img' && key === 'srcset') {
          this[key] = value = sanitizeSrcset(value, '$set(\'srcset\', value)');
        }

        if (writeAttr !== false) {
          if (value === null || isUndefined(value)) {
            this.$$element.removeAttr(attrName);
          } else {
            if (SIMPLE_ATTR_NAME.test(attrName)) {
              this.$$element.attr(attrName, value);
            } else {
              setSpecialAttr(this.$$element[0], attrName, value);
            }
          }
        }

        // fire observers
        var $$observers = this.$$observers;
        if ($$observers) {
          forEach($$observers[observer], function(fn) {
            try {
              fn(value);
            } catch (e) {
              $exceptionHandler(e);
            }
          });
        }
      }