When decorating a MarkerLayer, get its corresponding DisplayMarkerLayer

This fixes 'folded' line number decorations.

spec/text-editor-component-spec.js

@@ -224,6 +224,13 @@ describe('TextEditorComponent', () => {
     expect(scroller.clientWidth).toBe(scroller.scrollWidth)
   })
 
+  it('decorates the line numbers of folded lines', async () => {
+    const {component, element, editor} = buildComponent()
+    editor.foldBufferRow(1)
+    await component.getNextUpdatePromise()
+    expect(lineNumberNodeForScreenRow(component, 1).classList.contains('folded')).toBe(true)
+  })
+
   describe('focus', () => {
     it('focuses the hidden input element and adds the is-focused class when focused', async () => {
       assertDocumentFocused()

src/decoration-manager.js

@@ -200,6 +200,7 @@ class DecorationManager {
     if (markerLayer.isDestroyed()) {
       throw new Error('Cannot decorate a destroyed marker layer')
     }
+    markerLayer = this.displayLayer.getMarkerLayer(markerLayer.id)
     const decoration = new LayerDecoration(markerLayer, this, decorationParams)
     let layerDecorations = this.layerDecorationsByMarkerLayer.get(markerLayer)
     if (layerDecorations == null) {