Export Context type from types.ts instead of walkTree.ts.

When we stopped reexporting everything from walkTree.ts as part of #2533,
we also stopped exporting the Context type that it uses.

Moving the declaration and export of Context from walkTree.ts to types.ts
restores the ability to do

  import { Context } from "react-apollo"

which should fix #2824.

src/types.ts

@@ -12,6 +12,10 @@ import ApolloClient, {
 } from 'apollo-client';
 import { MutationFn } from './Mutation';
 
+export interface Context {
+  [key: string]: any;
+};
+
 export type OperationVariables = {
   [key: string]: any;
 };
@@ -31,7 +35,7 @@ export interface MutationOpts<TData = any, TGraphQLVariables = OperationVariable
   update?: MutationUpdaterFn<TData>;
   client?: ApolloClient<any>;
   notifyOnNetworkStatusChange?: boolean;
-  context?: Record<string, any>;
+  context?: Context;
   onCompleted?: (data: TData) => void;
   onError?: (error: ApolloError) => void;
   fetchPolicy?: FetchPolicy;
@@ -45,7 +49,7 @@ export interface QueryOpts<TGraphQLVariables = OperationVariables> {
   pollInterval?: number;
   client?: ApolloClient<any>;
   notifyOnNetworkStatusChange?: boolean;
-  context?: Record<string, any>;
+  context?: Context;
   partialRefetch?: boolean;
 }
 

src/walkTree.ts

@@ -1,8 +1,5 @@
 import * as React from 'react';
-
-export interface Context {
-  [key: string]: any;
-}
+import { Context } from './types';
 
 interface PreactElement<P> {
   attributes: P;