At Build this year, Microsoft announced IE Mode, which will open IE-only sites in an IE tab within the Edge browser. As spotted by Techdows, there's now a flag that you can activate in Edge Canary to turn on Internet Explorer integration.
While the flag is present in the latest Edge Dev build, it only seems to work in Edge Canary 188.8.131.52 and above. If you head over to edge://flags and search for 'Enable IE Integration', you can set it to Enable.
This will add a 'Show this page using Internet Explorer' option to the 'More tools' section of the menu. At this time, it only launches IE with the page you're trying to open, rather than including it in a tab within Edge. However, it seems to attempt opening it within Edge, displaying a message that your version of IE doesn't support IE mode yet, and that it needs to be updated.
Presumably, Microsoft will release an update for Internet Explorer to support the feature, and the ability to open the page in a separate window will be there for anyone running an older version of the browser.