move thunkfor out of class

src/exportMapBuilder.js

@@ -202,6 +202,16 @@ export function recursivePatternCapture(pattern, callback) {
   }
 }
 
+/**
+ * The creation of this closure is isolated from other scopes
+ * to avoid over-retention of unrelated variables, which has
+ * caused memory leaks. See #1266.
+ */
+function thunkFor(p, context) {
+  // eslint-disable-next-line no-use-before-define
+  return () => ExportMapBuilder.for(childContext(p, context));
+}
+
 export default class ExportMapBuilder {
   static get(source, context) {
     const path = resolve(source, context);
@@ -326,15 +336,6 @@ export default class ExportMapBuilder {
       return resolve.relative(value, path, context.settings);
     }
 
-    /**
-     * The creation of this closure is isolated from other scopes
-     * to avoid over-retention of unrelated variables, which has
-     * caused memory leaks. See #1266.
-     */
-    function thunkFor(p, context) {
-      return () => ExportMapBuilder.for(childContext(p, context));
-    }
-
     function processDynamicImport(source) {
       hasDynamicImports = true;
       if (source.type !== 'Literal') {