findBreakingChanges: replace 'localeCompare' with simplier alternative

Motivated by #2869

src/utilities/findBreakingChanges.js

@@ -4,6 +4,7 @@ import keyMap from '../jsutils/keyMap';
 import inspect from '../jsutils/inspect';
 import invariant from '../jsutils/invariant';
 
+import type { ObjectFieldNode } from '../language/ast';
 import { print } from '../language/printer';
 import { visit } from '../language/visitor';
 
@@ -541,16 +542,33 @@ function stringifyValue(value: mixed, type: GraphQLInputType): string {
 
   const sortedAST = visit(ast, {
     ObjectValue(objectNode) {
-      const fields = [...objectNode.fields].sort((fieldA, fieldB) =>
-        fieldA.name.value.localeCompare(fieldB.name.value),
-      );
+      // Make a copy since sort mutates array
+      const fields = [...objectNode.fields];
+
+      fields.sort(compareFieldNames);
       return { ...objectNode, fields };
     },
   });
 
   return print(sortedAST);
 }
 
+function compareFieldNames(
+  fieldA: ObjectFieldNode,
+  fieldB: ObjectFieldNode,
+): number {
+  const nameA = fieldA.name.value;
+  const nameB = fieldB.name.value;
+
+  if (nameA < nameB) {
+    return -1;
+  }
+  if (nameA > nameB) {
+    return 1;
+  }
+  return 0;
+}
+
 function diff<T: { name: string, ... }>(
   oldArray: $ReadOnlyArray<T>,
   newArray: $ReadOnlyArray<T>,