MediaList must not be shared between stylesheets

w3c/csswg-drafts/issues#11778

Author: cdoublev

__tests__/stylesheet.js

@@ -103,9 +103,8 @@ describe('CSSStyleSheet', () => {
         expect(styleSheet.media.mediaText).toBe(media.mediaText)
 
         media.mediaText = '(width)'
-        expect(styleSheet.media.mediaText).toBe('(width)')
-        styleSheet.media.mediaText = 'all'
-        expect(media.mediaText).toBe('all')
+        expect(media.mediaText).toBe('(width)')
+        expect(styleSheet.media.mediaText).toBe('all')
     })
     it('creates a non-constructed CSSStyleSheet', () => {
 

lib/cssom/CSSStyleSheet-impl.js

@@ -22,10 +22,9 @@ const UPDATE_LOCKED_STYLESHEET_ERROR = {
  */
 function createMediaList(list, globalObject) {
     if (MediaList.isImpl(list)) {
-        list = list._list
-    } else {
-        list = parseCSSGrammar(list, '<media-query-list>')
+        list = list.mediaText
     }
+    list = parseCSSGrammar(list, '<media-query-list>')
     return MediaList.createImpl(globalObject, undefined, { list })
 }