Update MenuPlacer context usage to the new context API

This solves #3916, and partially addresses #3703.
From React 16.3 the old Context API was deprecated and
it's rising warnings when runng in strict-mode.

packages/react-select/src/components/Menu.js

@@ -1,14 +1,14 @@
 // @flow
 /** @jsx jsx */
 import {
+  createContext,
   Component,
   type Element as ReactElement,
   type ElementRef,
   type Node,
 } from 'react';
 import { jsx } from '@emotion/core';
 import { createPortal } from 'react-dom';
-import PropTypes from 'prop-types';
 
 import {
   animatedScrollTo,
@@ -258,15 +258,16 @@ export const menuCSS = ({
   zIndex: 1,
 });
 
+const PortalPlacementContext = createContext(() => { });
+
 // NOTE: internal only
 export class MenuPlacer extends Component<MenuPlacerProps, MenuState> {
   state = {
     maxHeight: this.props.maxMenuHeight,
     placement: null,
   };
-  static contextTypes = {
-    getPortalPlacement: PropTypes.func,
-  };
+  static contextType = PortalPlacementContext;
+
   getPlacement = (ref: ElementRef<*>) => {
     const {
       minMenuHeight,
@@ -481,14 +482,6 @@ export const menuPortalCSS = ({ rect, offset, position }: PortalStyleArgs) => ({
 
 export class MenuPortal extends Component<MenuPortalProps, MenuPortalState> {
   state = { placement: null };
-  static childContextTypes = {
-    getPortalPlacement: PropTypes.func,
-  };
-  getChildContext() {
-    return {
-      getPortalPlacement: this.getPortalPlacement,
-    };
-  }
 
   // callback for occassions where the menu must "flip"
   getPortalPlacement = ({ placement }: MenuState) => {
@@ -526,6 +519,10 @@ export class MenuPortal extends Component<MenuPortalProps, MenuPortalState> {
       <div css={getStyles('menuPortal', state)}>{children}</div>
     );
 
-    return appendTo ? createPortal(menuWrapper, appendTo) : menuWrapper;
+    return (
+      <PortalPlacementContext.Provider value={this.getPortalPlacement}>
+        {appendTo ? createPortal(menuWrapper, appendTo) : menuWrapper}
+      </PortalPlacementContext.Provider>
+    );
   }
 }