Fix broken tests related to Edge 15 bugs with CSS Custom properties

Port webcomponents/shadycss#120 to 1.x

src/lib/settings.html

@@ -48,8 +48,10 @@
 
       // chrome 49 has semi-working css vars, check if box-shadow works
       // safari 9.1 has a recalc bug: https://bugs.webkit.org/show_bug.cgi?id=155782
+      // Edge 15: custom properties used in ::before and ::after will also be used in the parent element
+      // https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/12414257/
       settings.hasNativeCSSProperties =
-        (!navigator.userAgent.match('AppleWebKit/601') && window.CSS
+        (!navigator.userAgent.match(/AppleWebKit\/601|Edge\/15/) && window.CSS
         && CSS.supports && CSS.supports('box-shadow', '0 0 0 var(--foo)'));
 
       settings.useNativeCSSProperties = settings.hasNativeCSSProperties &&