Improved Flowtypes in response to code review feedback

src/renderers/shared/fiber/ReactFiberContext.js

@@ -37,7 +37,7 @@ if (__DEV__) {
 }
 
 let contextStackCursor : StackCursor<?Object> = createCursor((null: ?Object));
-let didPerformWorkStackCursor : StackCursor<?boolean> = createCursor((null: ?boolean));
+let didPerformWorkStackCursor : StackCursor<boolean> = createCursor(false);
 
 function getUnmaskedContext() {
   return contextStackCursor.current || emptyObject;
@@ -66,7 +66,7 @@ exports.getMaskedContext = function(workInProgress : Fiber) {
 };
 
 exports.hasContextChanged = function() : boolean {
-  return Boolean(didPerformWorkStackCursor.current);
+  return didPerformWorkStackCursor.current;
 };
 
 function isContextProvider(fiber : Fiber) : boolean {
@@ -136,8 +136,10 @@ exports.pushContextProvider = function(workInProgress : Fiber, didPerformWork :
 };
 
 exports.resetContext = function() : void {
-  reset(contextStackCursor);
-  reset(didPerformWorkStackCursor);
+  reset();
+
+  contextStackCursor.current = null;
+  didPerformWorkStackCursor.current = false;
 };
 
 exports.findCurrentUnmaskedContext = function(fiber: Fiber) : Object {

src/renderers/shared/fiber/ReactFiberHostContext.js

@@ -114,8 +114,10 @@ module.exports = function<T, P, I, TI, C, CX>(
   }
 
   function resetHostContainer() {
-    reset(contextStackCursor);
-    reset(rootInstanceStackCursor);
+    reset();
+
+    contextStackCursor.current = null;
+    rootInstanceStackCursor.current = null;
   }
 
   return {

src/renderers/shared/fiber/ReactFiberStack.js

@@ -55,9 +55,7 @@ exports.pop = function<T>(
     }
   }
 
-  cursor.current = index > 0
-    ? valueStack[index]
-    : (null : any);
+  cursor.current = valueStack[index];
 
   valueStack[index] = null;
 
@@ -70,7 +68,7 @@ exports.pop = function<T>(
 
 exports.push = function<T>(
   cursor : StackCursor<T>,
-  value : any,
+  value : T,
   fiber: Fiber,
 ) : void {
   index++;
@@ -84,9 +82,7 @@ exports.push = function<T>(
   cursor.current = value;
 };
 
-exports.reset = function<T>(
-  cursor : StackCursor<T>,
-) : void {
+exports.reset = function() : void {
   while (index > -1) {
     valueStack[index] = null;
 
@@ -96,6 +92,4 @@ exports.reset = function<T>(
 
     index--;
   }
-
-  cursor.current = (null : any);
 };