Feature detect Symbol support. Use string as a fallback.

CHANGELOG.md

@@ -96,6 +96,9 @@
 - Fully removed `prettier`. The Apollo Client team has decided to no longer automatically enforce code formatting across the codebase. In most cases existing code styles should be followed as much as possible, but this is not a hard and fast rule.  <br/>
   [@hwillson](https://github.com/hwillson) in [#5227](https://github.com/apollographql/apollo-client/pull/5227)
 
+- Make sure `ApolloContext` plays nicely with IE11 when storing the shared context.  <br/>
+  [@ms](https://github.com/ms) in [#5840](https://github.com/apollographql/apollo-client/pull/5840)
+
 ### Bug Fixes
 
 - `useMutation` adjustments to help avoid an infinite loop / too many renders issue, caused by unintentionally modifying the `useState` based mutation result directly.  <br/>

src/react/context/ApolloContext.ts

@@ -13,7 +13,11 @@ export interface ApolloContextValue {
 // Since the created context is React specific, we've decided to attach it to
 // the `React` object for sharing.
 
-const contextSymbol = Symbol.for('__APOLLO_CONTEXT__');
+// If Symbol's aren't available, we'll use a fallback string as the context
+// property (we're looking at you, IE11).
+const contextSymbol = typeof Symbol === 'function' ?
+  Symbol.for('__APOLLO_CONTEXT__') :
+  '__APOLLO_CONTEXT__';
 
 export function resetApolloContext() {
   const React = requireReactLazily();