Correctly mark email as optional

Look at the flows to see if there is one we can complete without
an email address. Mark the email address as optional iff there is.

src/components/structures/login/Registration.js

@@ -93,6 +93,7 @@ module.exports = React.createClass({
             doingUIAuth: Boolean(this.props.sessionId),
             hsUrl: this.props.customHsUrl,
             isUrl: this.props.customIsUrl,
+            flows: null,
         };
     },
 
@@ -145,11 +146,27 @@ module.exports = React.createClass({
         });
     },
 
-    _replaceClient: function() {
+    _replaceClient: async function() {
         this._matrixClient = Matrix.createClient({
             baseUrl: this.state.hsUrl,
             idBaseUrl: this.state.isUrl,
         });
+        try {
+            const result = await this._makeRegisterRequest({});
+            // This should never succeed since we specified an empty
+            // auth object.
+            console.log("Expecting 401 from register request but got success!");
+        } catch (e) {
+            if (e.httpStatus === 401) {
+                this.setState({
+                    flows: e.data.flows,
+                });
+            } else {
+                this.setState({
+                    errorText: _t("Unable to query for supported registration methods"),
+                });
+            }
+        }
     },
 
     onFormSubmit: function(formVals) {
@@ -378,7 +395,7 @@ module.exports = React.createClass({
                     poll={true}
                 />
             );
-        } else if (this.state.busy || this.state.teamServerBusy) {
+        } else if (this.state.busy || this.state.teamServerBusy || !this.state.flows) {
             registerBody = <Spinner />;
         } else {
             let serverConfigSection;
@@ -408,6 +425,7 @@ module.exports = React.createClass({
                         onError={this.onFormValidationFailed}
                         onRegisterClick={this.onFormSubmit}
                         onTeamSelected={this.onTeamSelected}
+                        flows={this.state.flows}
                     />
                     { serverConfigSection }
                 </div>