Create graphQLUpload schema type

angel_graphql/lib/angel_graphql.dart

@@ -1,4 +1,46 @@
+import 'package:angel_framework/angel_framework.dart';
+import 'package:graphql_schema/graphql_schema.dart';
 export 'src/graphiql.dart';
 export 'src/graphql_http.dart';
 export 'src/graphql_ws.dart';
 export 'src/resolvers.dart';
+
+/// The canonical [GraphQLUploadType] instance.
+final GraphQLUploadType graphQLUpload = GraphQLUploadType();
+
+/// A [GraphQLScalarType] that is used to read uploaded files from
+/// `multipart/form-data` requests.
+class GraphQLUploadType extends GraphQLScalarType<UploadedFile, UploadedFile> {
+  @override
+  String get name => 'Upload';
+
+  @override
+  String get description =>
+      'Represents a file that has been uploaded to the server.';
+
+  @override
+  GraphQLType<UploadedFile, UploadedFile> coerceToInputObject() => this;
+
+  @override
+  UploadedFile deserialize(UploadedFile serialized) => serialized;
+
+  @override
+  UploadedFile serialize(UploadedFile value) => value;
+
+  @override
+  ValidationResult<UploadedFile> validate(String key, UploadedFile input) {
+    if (input != null && input is! UploadedFile) {
+      return _Vr(false, errors: ['Expected "$key" to be a boolean.']);
+    }
+    return _Vr(true, value: input);
+  }
+}
+
+// TODO: Really need to make the validation result constructors *public*
+class _Vr<T> implements ValidationResult<T> {
+  final bool successful;
+  final List<String> errors;
+  final T value;
+
+  _Vr(this.successful, {this.errors, this.value});
+}