Account for the deprecation of ValidationContext.prototype.getErrors.

We'll use the new `onError` callback which was introduced in
`graphql@14.5.0` by graphql/graphql-js#2074 only if
the `getErrors` method that existed prior to that addition no longer exists.

packages/apollo-language-server/src/errors/validation.ts

@@ -51,7 +51,18 @@ export function getValidationErrors(
   rules: ValidationRule[] = defaultValidationRules
 ) {
   const typeInfo = new TypeInfo(schema);
-  const context = new ValidationContext(schema, document, typeInfo);
+
+  // The 4th argument to `ValidationContext` is an `onError` callback. This was
+  // introduced by https://github.com/graphql/graphql-js/pull/2074 and first
+  // published in graphql@14.5.0. It is meant to replace the `getErrors` method
+  // which was previously used. Since we support versions of graphql older than
+  // that, it's possible that this callback will not be invoked and we'll need
+  // to resort to using `getErrors`. Therefore, although we'll collect errors
+  // via this callback, if `getErrors` is present on the context we create,
+  // we'll go ahead and use that instead.
+  const errors: GraphQLError[] = [];
+  const onError = (err: GraphQLError) => errors.push(err);
+  const context = new ValidationContext(schema, document, typeInfo, onError);
 
   if (fragments) {
     (context as any)._fragments = fragments;
@@ -60,7 +71,14 @@ export function getValidationErrors(
   const visitors = rules.map(rule => rule(context));
   // Visit the whole document with each instance of all provided rules.
   visit(document, visitWithTypeInfo(typeInfo, visitInParallel(visitors)));
-  return context.getErrors();
+
+  // @ts-ignore
+  // `getErrors` is gone in `graphql@15`, but we still support older versions.
+  if (typeof context.getErrors === "function") return context.getErrors();
+
+  // If `getErrors` doesn't exist, we must be on a `graphql@15` or higher,
+  // so we'll use the errors we collected via the `onError` callback.
+  return errors;
 }
 
 export function validateQueryDocument(