update schema and fix import error

build/vega-lite-schema.json

@@ -976,7 +976,7 @@
               "type": "null"
             }
           ],
-          "description": "Sort order for the encoded field.\nSupported `sort` values include `\"ascending\"`, `\"descending\"`, `\"array specifying the preferred order of values\"` and `null` (no sorting).\nFor fields with discrete domains, `sort` can also be a [sort field definition object](https://vega.github.io/vega-lite/docs/sort.html#sort-field).\nFor `sort` as an \"array specifying the preferred order of values\", the sort order will obey the values in the array, followed by any unspecified values in their original order.\n\n__Default value:__ `\"ascending\"`"
+          "description": "Sort order for the encoded field.\nSupported `sort` values include `\"ascending\"`, `\"descending\"`, `array specifying the preferred order of values` and `null` (no sorting).\nFor fields with discrete domains, `sort` can also be a [sort field definition object](https://vega.github.io/vega-lite/docs/sort.html#sort-field).\nFor `sort` as an [array specifying the preferred order of values](https://vega.github.io/vega-lite/docs/sort.html#sort-array), the sort order will obey the values in the array, followed by any unspecified values in their original order.\n\n__Default value:__ `\"ascending\"`"
         },
         "test": {
           "$ref": "#/definitions/LogicalOperand<Predicate>"
@@ -1182,7 +1182,7 @@
               "type": "null"
             }
           ],
-          "description": "Sort order for the encoded field.\nSupported `sort` values include `\"ascending\"`, `\"descending\"`, `\"array specifying the preferred order of values\"` and `null` (no sorting).\nFor fields with discrete domains, `sort` can also be a [sort field definition object](https://vega.github.io/vega-lite/docs/sort.html#sort-field).\nFor `sort` as an \"array specifying the preferred order of values\", the sort order will obey the values in the array, followed by any unspecified values in their original order.\n\n__Default value:__ `\"ascending\"`"
+          "description": "Sort order for the encoded field.\nSupported `sort` values include `\"ascending\"`, `\"descending\"`, `array specifying the preferred order of values` and `null` (no sorting).\nFor fields with discrete domains, `sort` can also be a [sort field definition object](https://vega.github.io/vega-lite/docs/sort.html#sort-field).\nFor `sort` as an [array specifying the preferred order of values](https://vega.github.io/vega-lite/docs/sort.html#sort-array), the sort order will obey the values in the array, followed by any unspecified values in their original order.\n\n__Default value:__ `\"ascending\"`"
         },
         "timeUnit": {
           "$ref": "#/definitions/TimeUnit",
@@ -2153,7 +2153,7 @@
               "type": "null"
             }
           ],
-          "description": "Sort order for the encoded field.\nSupported `sort` values include `\"ascending\"`, `\"descending\"`, `\"array specifying the preferred order of values\"` and `null` (no sorting).\nFor fields with discrete domains, `sort` can also be a [sort field definition object](https://vega.github.io/vega-lite/docs/sort.html#sort-field).\nFor `sort` as an \"array specifying the preferred order of values\", the sort order will obey the values in the array, followed by any unspecified values in their original order.\n\n__Default value:__ `\"ascending\"`"
+          "description": "Sort order for the encoded field.\nSupported `sort` values include `\"ascending\"`, `\"descending\"`, `array specifying the preferred order of values` and `null` (no sorting).\nFor fields with discrete domains, `sort` can also be a [sort field definition object](https://vega.github.io/vega-lite/docs/sort.html#sort-field).\nFor `sort` as an [array specifying the preferred order of values](https://vega.github.io/vega-lite/docs/sort.html#sort-array), the sort order will obey the values in the array, followed by any unspecified values in their original order.\n\n__Default value:__ `\"ascending\"`"
         },
         "timeUnit": {
           "$ref": "#/definitions/TimeUnit",
@@ -4260,7 +4260,7 @@
               "type": "null"
             }
           ],
-          "description": "Sort order for the encoded field.\nSupported `sort` values include `\"ascending\"`, `\"descending\"`, `\"array specifying the preferred order of values\"` and `null` (no sorting).\nFor fields with discrete domains, `sort` can also be a [sort field definition object](https://vega.github.io/vega-lite/docs/sort.html#sort-field).\nFor `sort` as an \"array specifying the preferred order of values\", the sort order will obey the values in the array, followed by any unspecified values in their original order.\n\n__Default value:__ `\"ascending\"`"
+          "description": "Sort order for the encoded field.\nSupported `sort` values include `\"ascending\"`, `\"descending\"`, `array specifying the preferred order of values` and `null` (no sorting).\nFor fields with discrete domains, `sort` can also be a [sort field definition object](https://vega.github.io/vega-lite/docs/sort.html#sort-field).\nFor `sort` as an [array specifying the preferred order of values](https://vega.github.io/vega-lite/docs/sort.html#sort-array), the sort order will obey the values in the array, followed by any unspecified values in their original order.\n\n__Default value:__ `\"ascending\"`"
         },
         "stack": {
           "anyOf": [

src/sort.ts

@@ -1,6 +1,6 @@
-import {AggregateOp} from './aggregate';
+import {AggregateOp} from 'vega';
+import {isArray, isString} from 'vega-util';
 import {SortField} from './sort';
-import {isArray, isString} from './util';
 
 export type SortOrder = 'ascending' | 'descending' | null;