Conditionally import PossibleTypeExtensions validation rule (#1935)

As described in apollographql/apollo-server#4066,
older versions of `graphql` do not have this type.

Co-authored-by: Jesse Rosenberger <git@jro.cc>

packages/apollo-graphql/src/schema/buildSchemaFromSDL.ts

@@ -27,24 +27,36 @@ import { GraphQLSchemaValidationError } from "./GraphQLSchemaValidationError";
 import { specifiedSDLRules } from "graphql/validation/specifiedRules";
 import {
   KnownTypeNamesRule,
-  UniqueDirectivesPerLocationRule
+  UniqueDirectivesPerLocationRule,
+  ValidationRule
 } from "graphql/validation";
-// Currently, this PossibleTypeExtensions rule is experimental and thus not
-// exposed directly from the rules module above. This may change in the future!
-import { PossibleTypeExtensions } from "graphql/validation/rules/PossibleTypeExtensions";
 import { mapValues, isNotNullOrUndefined } from "apollo-env";
 
 export interface GraphQLSchemaModule {
   typeDefs: DocumentNode;
   resolvers?: GraphQLResolverMap<any>;
 }
 
-const skippedSDLRules = [
-  PossibleTypeExtensions,
+const skippedSDLRules: ValidationRule[] = [
   KnownTypeNamesRule,
   UniqueDirectivesPerLocationRule
 ];
 
+// BREAKING VERSION: Remove this when graphql-js 15 is minimum version.
+// Currently, this PossibleTypeExtensions rule is experimental and thus not
+// exposed directly from the rules module above. This may change in the future!
+// Additionally, it does not exist in prior graphql versions. Thus this try/catch.
+try {
+  const PossibleTypeExtensions: typeof import("graphql/validation/rules/PossibleTypeExtensions").PossibleTypeExtensions = require("graphql/validation/rules/PossibleTypeExtensions")
+    .PossibleTypeExtensions;
+  if (PossibleTypeExtensions) {
+    skippedSDLRules.push(PossibleTypeExtensions);
+  }
+} catch (e) {
+  // No need to fail in this case.  Instead, if this validation rule is missing, we will assume its not used
+  // by the version of `graphql` that is available to us.
+}
+
 const sdlRules = specifiedSDLRules.filter(
   rule => !skippedSDLRules.includes(rule)
 );