Update flow version and address type errors. (#1369)

src/defineEnvironment.js

@@ -110,6 +110,10 @@ export default function defineEnvironment<NODETYPE: NodeType>({
         handler,
     };
     for (let i = 0; i < names.length; ++i) {
+        // TODO: The value type of _environments should be a type union of all
+        // possible `EnvSpec<>` possibilities instead of `EnvSpec<*>`, which is
+        // an existential type.
+        // $FlowFixMe
         _environments[names[i]] = data;
     }
     if (htmlBuilder) {