Fix sorting of side-effect imports in Node <12

`Array.prototype.sort` in Node <12 does not use stable sorting, and this
plugin accidentally relied on stable sorting to keep the original order
of side-effect imports. This commit makes sure that side effect imports
end up in their original order regardless of whether `.sort` is stable
or not.

Fixes #34 and closes #36.

README.md

@@ -634,7 +634,7 @@ easier to work with.
 
 ## Development
 
-You can need [Node.js] 10 and npm 6.
+You need [Node.js] ~12 and npm 6.
 
 ### npm scripts
 

src/sort.js

@@ -776,7 +776,7 @@ function sortImportItems(items) {
   return items.slice().sort((itemA, itemB) =>
     // If both items are side effect imports, keep their original order.
     itemA.isSideEffectImport && itemB.isSideEffectImport
-      ? 0
+      ? itemA.index - itemB.index
       : // If one of the items is a side effect import, move it first.
       itemA.isSideEffectImport
       ? -1

test/sort.test.js

@@ -100,6 +100,21 @@ const baseTests = expect => ({
           |import "a"
     `,
 
+    // Side-effect only imports use a stable sort (issure #34).
+    input`
+          |import "codemirror/addon/fold/brace-fold"
+          |import "codemirror/addon/edit/closebrackets"
+          |import "codemirror/addon/fold/foldgutter"
+          |import "codemirror/addon/fold/foldgutter.css"
+          |import "codemirror/addon/lint/json-lint"
+          |import "codemirror/addon/lint/lint"
+          |import "codemirror/addon/lint/lint.css"
+          |import "codemirror/addon/scroll/simplescrollbars"
+          |import "codemirror/addon/scroll/simplescrollbars.css"
+          |import "codemirror/lib/codemirror.css"
+          |import "codemirror/mode/javascript/javascript"
+    `,
+
     // Sorted alphabetically.
     input`
           |import x1 from "a";