[fixed] respect closeTimeoutMS during unmount

Fixes issue reactjs#17: "When the modal is unmounted, it will abruptly close,
not waiting for any animations to finish."

The bug was caused by the Modal component unmounting the portal
immediately in `componentWillUnmount` regardless of whether the portal
is currently closing or would animate to close.

Now when a Modal has a non-zero closeTimeoutMS, the Modal inspects the
portal state before closing.  If the portal is open or closing, but not
closed, it waits to unmount the portal.  If the portal is open and not
already closing, the Modal calls closeWithTimeout() to trigger the
close.

lib/components/Modal.js

@@ -105,6 +105,24 @@ export default class Modal extends Component {
       ariaAppHider.show(this.props.appElement);
     }
 
+    var state = this.portal.state;
+    var now = Date.now();
+    var closesAt = state.isOpen && this.props.closeTimeoutMS
+      && (state.closesAt
+        || now + this.props.closeTimeoutMS);
+
+    if (closesAt) {
+      if (!state.beforeClose) {
+        this.portal.closeWithTimeout();
+      }
+
+      setTimeout(this.removePortal, closesAt - now);
+    } else {
+      this.removePortal();
+    }
+  }
+
+  removePortal () {
     ReactDOM.unmountComponentAtNode(this.node);
     const parent = getParentElement(this.props.parentSelector);
     parent.removeChild(this.node);

lib/components/ModalPortal.js

@@ -132,16 +132,18 @@ export default class ModalPortal extends Component {
   }
 
   closeWithTimeout () {
-    this.setState({ beforeClose: true }, () => {
-      this.closeTimer = setTimeout(this.closeWithoutTimeout, this.props.closeTimeoutMS);
-    });
+    var closesAt = Date.now() + this.props.closeTimeoutMS;
+    this.setState({beforeClose: true, closesAt: closesAt}, function() {
+      this.closeTimer = setTimeout(this.closeWithoutTimeout, this.state.closesAt - Date.now());
+    }.bind(this));
   }
 
   closeWithoutTimeout = () => {
     this.setState({
       beforeClose: false,
       isOpen: false,
       afterOpen: false,
+      closesAt: null
     }, this.afterClose);
   }
 
@@ -182,7 +184,7 @@ export default class ModalPortal extends Component {
   }
 
   shouldBeClosed () {
-    return !this.props.isOpen && !this.state.beforeClose;
+    return !this.state.isOpen && !this.state.beforeClose;
   }
 
   contentHasFocus () {