Include fragment name lookahead in parser as specced

Author: leebyron

src/language/__tests__/parser.js

@@ -63,6 +63,12 @@ fragment MissingOn Type
     ).to.throw('Syntax Error GraphQL (1:22) Duplicate input object field a.');
   });
 
+  it('does not accept fragments named "on"', () => {
+    expect(
+      () => parse('fragment on on on { on }')
+    ).to.throw('Syntax Error GraphQL (1:10) Unexpected Name "on"');
+  });
+
   var kitchenSink = readFileSync(
     join(__dirname, '/kitchen-sink.graphql'),
     { encoding: 'utf8' }

src/language/parser.js

@@ -435,18 +435,25 @@ function parseFragment(parser): FragmentSpread | InlineFragment {
   }
   return {
     kind: FRAGMENT_SPREAD,
-    name: parseName(parser),
+    name: parseFragmentName(parser),
     directives: parseDirectives(parser),
     loc: loc(parser, start)
   };
 }
 
+function parseFragmentName(parser): Name {
+  if (parser.token.value === 'on') {
+    throw unexpected(parser);
+  }
+  return parseName(parser);
+}
+
 function parseFragmentDefinition(parser): FragmentDefinition {
   var start = parser.token.start;
   expectKeyword(parser, 'fragment');
   return {
     kind: FRAGMENT_DEFINITION,
-    name: parseName(parser),
+    name: parseFragmentName(parser),
     typeCondition: (expectKeyword(parser, 'on'), parseName(parser)),
     directives: parseDirectives(parser),
     selectionSet: parseSelectionSet(parser),