Update ShExUtil.js
Update ShExUtil.js

`shapeExprs` and `tripleExprs` were declared to be [Maps](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map), but they're populated (and accessed throughout shex.js) with object properties - `index.tripleExprs[expression.id] = expression` just sets a regular object property. If you change it to actually set a map entry with `index.tripleExprs.set(expression.id, expression)` then you'll have to change all the usages of `index.tripleExprs[expr]` to `index.tripleExprs.get(expr)` and all the usages of `Object.keys(index.tripleExprs)` to use iterators like `index.tripleExprs.keys()` or `index.tripleExprs.entries()`, etc.
shexSpec/shex.js,shexSpec/shex.js,shexSpec/shex.js