Fix making https://github.com/bokeh/bokeh/pull/9153 build successfully

hover.py

"""Example demonstrating hovering with line widths specified in the data source - with bokeh server"""

from bokeh.io import curdoc
from bokeh.plotting import figure

data = dict(
    x=[0, 0, 25],
    y=[5, 20, 5],
    dw=[20,  20, 10],
    dh=[10,  10, 25],
    colors=['blue', 'blue', 'blue'],
    alphas=[0.2, 0.4, 0.6],
    hover_line_widths=[5, 10, 15],
)

p = figure(x_range=(0, 35), y_range=(0, 35), tools='hover')

p.rect(source=data,
       x='x', y='y',
       width='dw', height='dh',
       color='colors',
       alpha='alphas',
       hover_line_width='hover_line_widths'
       )

curdoc().add_root(p)

plotting.js

diff --git a/bokehjs/src/lib/api/plotting.ts b/bokehjs/src/lib/api/plotting.ts
index a42a95acd..0eb36a718 100644
--- a/bokehjs/src/lib/api/plotting.ts
+++ b/bokehjs/src/lib/api/plotting.ts
@@ -631,7 +631,7 @@ export class Figure extends Plot {
   }

   _pop_visuals(cls: Class<HasProps>, props: Attrs, prefix: string = "",
-                        defaults: Attrs = {}, trait_defaults: Attrs = {}): Attrs {
+                        defaults_: Attrs = {}, trait_defaults_: Attrs = {}): Attrs {

     const _split_feature_trait = function(ft: string): string {
       ft = ft.split('_', 2)
@@ -643,25 +643,27 @@ export class Figure extends Plot {
       return ['line', 'fill', 'text'].includes(feature) && (trait != null)
     }

-    const defaults : Attrs = {...defaults}
+    const defaults : Attrs = {...defaults_}
     if ( !defaults.includes('text_color') ) {
       defaults['text_color'] = 'black' }
-    const trait_defaults : Attrs = {...trait_defaults}
+    const trait_defaults : Attrs = {...trait_defaults_}
     if ( !trait_defaults.includes('color') ) {
       trait_defaults['color'] = _default_color }
     if ( !trait_defaults.includes('alpha') ){
       trait_defaults['alpha'] = _default_alpha }

     const result: Attrs = {}
-    traits = new Set()
-    for (pname in cls.properties) {
+    const traits = new Set()
+    for (const pname in cls.properties) {
       if (_is_visual(pname)) {
-        [_, trait] = _split_feature_trait(pname)
+        const [_, trait] = _split_feature_trait(pname)
         if (props.includes(prefix+pname)) {
             result[pname] = props[prefix+pname]
-            delete props[prefix+pname] }
+            delete props[prefix+pname]
+        }
         else if (props.includes(prefix+trait)) {
-            result[pname] = props[prefix+trait] }
+            result[pname] = props[prefix+trait]
+        }
         else if (defaults.includes(pname)) {
             result[pname] = defaults[pname] }
         else if (trait_defaults.includes(trait)) {

plotting.js goes to bokehjs/src/lib/api/
hover.py goes to examples/app/
Run example by invoking python -m bokeh serve --show examples/app/hover.py

/cc @mattpap