change execute implementation for async iterable resolvers

# Conflicts:
#	src/execution/execute.js

Author: robrichard

src/execution/execute.js

@@ -867,37 +867,57 @@ function completeAsyncIteratorValue(
   fieldNodes: $ReadOnlyArray<FieldNode>,
   info: GraphQLResolveInfo,
   path: Path,
-  index: number,
-  completedResults: Array<mixed>,
   iterator: AsyncIterator<mixed>,
 ): Promise<$ReadOnlyArray<mixed>> {
-  const fieldPath = addPath(path, index, undefined);
-  return iterator.next().then(
-    ({ value, done }) => {
-      if (done) {
-        return completedResults;
-      }
-      completedResults.push(
-        completeValue(exeContext, itemType, fieldNodes, info, fieldPath, value),
-      );
-      return completeAsyncIteratorValue(
-        exeContext,
-        itemType,
-        fieldNodes,
-        info,
-        path,
-        index + 1,
-        completedResults,
-        iterator,
+  return new Promise((resolve) => {
+    function next(index, completedResults) {
+      const fieldPath = addPath(path, index, undefined);
+      iterator.next().then(
+        ({ value, done }) => {
+          if (done) {
+            resolve(completedResults);
+            return;
+          }
+          // TODO can the error checking logic be consolidated with completeListValue?
+          try {
+            completedResults.push(
+              completeValue(
+                exeContext,
+                itemType,
+                fieldNodes,
+                info,
+                fieldPath,
+                value,
+              ),
+            );
+          } catch (rawError) {
+            completedResults.push(null);
+            const error = locatedError(
+              rawError,
+              fieldNodes,
+              pathToArray(fieldPath),
+            );
+            handleFieldError(error, itemType, exeContext);
+            resolve(completedResults);
+            return;
+          }
+
+          next(index + 1, completedResults);
+        },
+        (rawError) => {
+          completedResults.push(null);
+          const error = locatedError(
+            rawError,
+            fieldNodes,
+            pathToArray(fieldPath),
+          );
+          handleFieldError(error, itemType, exeContext);
+          resolve(completedResults);
+        },
       );
-    },
-    (rawError) => {
-      completedResults.push(null);
-      const error = locatedError(rawError, fieldNodes, pathToArray(fieldPath));
-      handleFieldError(error, itemType, exeContext);
-      return completedResults;
-    },
-  );
+    }
+    next(0, []);
+  });
 }
 
 /**
@@ -923,8 +943,6 @@ function completeListValue(
       fieldNodes,
       info,
       path,
-      0,
-      [],
       iterator,
     );
   }