compat: filter extensions prior to passing to buildASTSchema.

.@IvanGoncharov originally brought this to my attention when he pointed me to
yaacovCR#32 (comment)
and suggested stripping extension nodes prior to invoking `buildASTSchema`
as a cross-version (v14 <=> v15) approach for interim compatibility on the
v4 series of `graphql-tools`.

The most urgent and pertinent need here from my perspective is to allow
user-exploration of the new `graphql@15` release candidate within Apollo
Server which currently re-exports the entirety of `graphql-tools` (even
though it only relies on small portions of it).

Upon further investigation of the above-referenced issue, it appears that
@yaacovCR had already crafted the solution that @IvanGoncharov had suggested
to me, which I found in 2280eef within the
well-organized #1206
(which I am thankful for the continued updates on!).

My commit here merely grabs a sub-set of that commit that seemed most
pertinent; I certainly don't claim that this solution is nearly as
comprehensive as the original 2280eef.  My
hope is that by using the same code/implementation here, it will marginally
lessen future merge conflicts.

Since this is basically a re-working of @yaacovCR's commit, I've attributed
co-authorship of this commit accordingly.  (Thank you, again!)

Ref: #1272
Co-authored-by: yaacovCR <yaacovCR@gmail.com>

src/generate/buildSchemaFromTypeDefinitions.ts

@@ -12,6 +12,7 @@ import {
   concatenateTypeDefs,
   SchemaError,
 } from '.';
+import filterExtensionDefinitions from './filterExtensionDefinitions';
 
 function buildSchemaFromTypeDefinitions(
   typeDefinitions: ITypeDefinitions,
@@ -38,10 +39,11 @@ function buildSchemaFromTypeDefinitions(
   }
 
   const backcompatOptions = { commentDescriptions: true };
+  const typesAst = filterExtensionDefinitions(astDocument);
 
   // TODO fix types https://github.com/apollographql/graphql-tools/issues/542
   let schema: GraphQLSchema = (buildASTSchema as any)(
-    astDocument,
+    typesAst,
     backcompatOptions,
   );
 

src/generate/filterExtensionDefinitions.ts

@@ -0,0 +1,19 @@
+import { DocumentNode, DefinitionNode, Kind } from 'graphql';
+
+export default function filterExtensionDefinitions(ast: DocumentNode) {
+  const extensionDefs = ast.definitions.filter(
+    (def: DefinitionNode) =>
+      def.kind !== Kind.OBJECT_TYPE_EXTENSION &&
+      def.kind !== Kind.INTERFACE_TYPE_EXTENSION &&
+      def.kind !== Kind.INPUT_OBJECT_TYPE_EXTENSION &&
+      def.kind !== Kind.UNION_TYPE_EXTENSION &&
+      def.kind !== Kind.ENUM_TYPE_EXTENSION &&
+      def.kind !== Kind.SCALAR_TYPE_EXTENSION &&
+      def.kind !== Kind.SCHEMA_EXTENSION,
+  );
+
+  return {
+    ...ast,
+    definitions: extensionDefs,
+  };
+}