[Experimental] Subscription queries

This simply adds `subscription` as an operation kind to the parser and AST. No semantics or implementations are added here.

This should unlock those who are beginning experimentation with GraphQL subscriptions.

Author: leebyron

src/language/__tests__/parser.js

@@ -168,6 +168,14 @@ fragment ${fragmentName} on Type {
     });
   });
 
+  it('parses experimental subscription feature', () => {
+    expect(() => parse(`
+      subscription Foo {
+        subscriptionField
+      }
+    `)).to.not.throw();
+  });
+
   it('parse creates ast', () => {
 
     var source = new Source(`{

src/language/ast.js

@@ -81,7 +81,8 @@ export type Definition = OperationDefinition
 export type OperationDefinition = {
   kind: 'OperationDefinition';
   loc?: ?Location;
-  operation: 'query' | 'mutation';
+  // Note: subscription is an experimental non-spec addition.
+  operation: 'query' | 'mutation' | 'subscription';
   name?: ?Name;
   variableDefinitions?: ?Array<VariableDefinition>;
   directives?: ?Array<Directive>;

src/language/parser.js

@@ -191,7 +191,9 @@ function parseDefinition(parser): Definition {
   if (peek(parser, TokenKind.NAME)) {
     switch (parser.token.value) {
       case 'query':
-      case 'mutation': return parseOperationDefinition(parser);
+      case 'mutation':
+      // Note: subscription is an experimental non-spec addition.
+      case 'subscription': return parseOperationDefinition(parser);
 
       case 'fragment': return parseFragmentDefinition(parser);