Bug 1148893 - Replace .then handlers with yields to catch 'Unknown sheet source' rejections in Style Editor onShow task. r=ejpbruel

browser/devtools/styleeditor/StyleEditorUI.jsm

@@ -550,13 +550,14 @@ StyleEditorUI.prototype = {
           this.emit("editor-selected", editor);
 
           // Is there any CSS coverage markup to include?
-          csscoverage.getUsage(this._target).then(usage => {
+          let usage = yield csscoverage.getUsage(this._target);
           if (usage == null) {
             return;
           }
 
           let href = csscoverage.sheetToUrl(editor.styleSheet);
-            usage.createEditorReport(href).then(data => {
+          let data = yield usage.createEditorReport(href)
+
           editor.removeAllUnusedRegions();
 
           if (data.reports.length > 0) {
@@ -572,8 +573,6 @@ StyleEditorUI.prototype = {
               this.emit("error", { key: "error-compressed", level: "info" });
             }
           }
-            }, Cu.reportError);
-          }, Cu.reportError);
         }.bind(this)).then(null, Cu.reportError);
       }.bind(this)
     });