Simplify getDirectiveNames and remove flattenSelections helper.

Though it's possible that someone might have been using flattenSelections
directly, the function signature seems awfully specific to the needs of
the former implementation of getDirectiveNames.

CHANGELOG.md

@@ -37,6 +37,24 @@
   GraphQL AST's are no longer being directly modified.  <br/>
   [@hwillson](https://github.com/hwillson) in [#4233](https://github.com/apollographql/apollo-client/pull/4233)
 
+- The `flattenSelections` helper function is no longer exported from
+  `apollo-utilities`, since `getDirectiveNames` has been reimplemented
+  without using `flattenSelections`, and `flattenSelections` has no clear
+  purpose now. If you need the old functionality, use a visitor:
+  ```ts
+  import { visit } from "graphql/language/visitor";
+
+  function flattenSelections(selection: SelectionNode) {
+    const selections: SelectionNode[] = [];
+    visit(selection, {
+      SelectionSet(ss) {
+        selections.push(...ss.selections);
+      }
+    });
+    return selections;
+  }
+  ```
+
 ### Apollo Cache In-Memory (vNext)
 
 - Export the optimism `wrap` function using ES2015 export syntax, instead of

packages/apollo-utilities/src/directives.ts

@@ -2,14 +2,15 @@
 // the `skip` and `include` directives within GraphQL.
 import {
   FieldNode,
-  OperationDefinitionNode,
   SelectionNode,
   VariableNode,
   BooleanValueNode,
   DirectiveNode,
   DocumentNode,
 } from 'graphql';
 
+import { visit } from 'graphql/language/visitor';
+
 import { argumentsObjectFromField } from './storeUtils';
 
 export type DirectiveInfo = {
@@ -95,45 +96,16 @@ export function shouldInclude(
   return res;
 }
 
-export function flattenSelections(selection: SelectionNode): SelectionNode[] {
-  if (
-    !(selection as FieldNode).selectionSet ||
-    !((selection as FieldNode).selectionSet.selections.length > 0)
-  )
-    return [selection];
-
-  return [selection].concat(
-    (selection as FieldNode).selectionSet.selections
-      .map(selectionNode =>
-        [selectionNode].concat(flattenSelections(selectionNode)),
-      )
-      .reduce((selections, selected) => selections.concat(selected), []),
-  );
-}
-
 export function getDirectiveNames(doc: DocumentNode) {
-  // operation => [names of directives];
-  const directiveNames = doc.definitions
-    .filter(
-      (definition: OperationDefinitionNode) =>
-        definition.selectionSet && definition.selectionSet.selections,
-    )
-    // operation => [[Selection]]
-    .map(x => flattenSelections(x as any))
-    // [[Selection]] => [Selection]
-    .reduce((selections, selected) => selections.concat(selected), [])
-    // [Selection] => [Selection with Directives]
-    .filter(
-      (selection: SelectionNode) =>
-        selection.directives && selection.directives.length > 0,
-    )
-    // [Selection with Directives] => [[Directives]]
-    .map((selection: SelectionNode) => selection.directives)
-    // [[Directives]] => [Directives]
-    .reduce((directives, directive) => directives.concat(directive), [])
-    // [Directives] => [Name]
-    .map((directive: DirectiveNode) => directive.name.value);
-  return directiveNames;
+  const names: string[] = [];
+
+  visit(doc, {
+    Directive(node) {
+      names.push(node.name.value);
+    },
+  });
+
+  return names;
 }
 
 export function hasDirectives(names: string[], doc: DocumentNode) {