Use global flag to optimize checking context dependencies

Track whether *any* contexts have changed on the stack. If this flag is
false, we can skip checking any context dependencies. Since it's unusual
that more than one context updates within the same batch, this should
save us a lot of unnecessary checks.

packages/react-reconciler/src/ReactFiberNewContext.js

@@ -28,7 +28,7 @@ import {ContextProvider} from 'shared/ReactTypeOfWork';
 
 import invariant from 'shared/invariant';
 
-const providerCursor: StackCursor<Fiber | null> = createCursor(null);
+const someContextChangedCursor: StackCursor<boolean> = createCursor(false);
 const valueCursor: StackCursor<mixed> = createCursor(null);
 const changedBitsCursor: StackCursor<number> = createCursor(0);
 
@@ -48,10 +48,16 @@ export function pushProvider(providerFiber: Fiber): void {
   if (isPrimaryRenderer) {
     push(changedBitsCursor, context._changedBits, providerFiber);
     push(valueCursor, context._currentValue, providerFiber);
-    push(providerCursor, providerFiber, providerFiber);
 
     context._currentValue = providerFiber.pendingProps.value;
-    context._changedBits = providerFiber.stateNode;
+    const changedBits = (context._changedBits = providerFiber.stateNode);
+
+    push(
+      someContextChangedCursor,
+      someContextChangedCursor.current || changedBits !== 0,
+      providerFiber,
+    );
+
     if (__DEV__) {
       warning(
         context._currentRenderer === undefined ||
@@ -65,10 +71,16 @@ export function pushProvider(providerFiber: Fiber): void {
   } else {
     push(changedBitsCursor, context._changedBits2, providerFiber);
     push(valueCursor, context._currentValue2, providerFiber);
-    push(providerCursor, providerFiber, providerFiber);
 
     context._currentValue2 = providerFiber.pendingProps.value;
-    context._changedBits2 = providerFiber.stateNode;
+    const changedBits = (context._changedBits2 = providerFiber.stateNode);
+
+    push(
+      someContextChangedCursor,
+      someContextChangedCursor.current || changedBits !== 0,
+      providerFiber,
+    );
+
     if (__DEV__) {
       warning(
         context._currentRenderer2 === undefined ||
@@ -86,7 +98,7 @@ export function popProvider(providerFiber: Fiber): void {
   const changedBits = changedBitsCursor.current;
   const currentValue = valueCursor.current;
 
-  pop(providerCursor, providerFiber);
+  pop(someContextChangedCursor, providerFiber);
   pop(valueCursor, providerFiber);
   pop(changedBitsCursor, providerFiber);
 
@@ -207,6 +219,12 @@ export function prepareToReadContext(
     // Reset the work-in-progress list
     workInProgress.firstContextDependency = null;
 
+    // Check the global flag to see if there are any pending contexts. This is
+    // optimizes for the normal case where no contexts have updated at all.
+    if (someContextChangedCursor.current === false) {
+      return false;
+    }
+
     // Iterate through the context dependencies and see if there were any
     // changes. If so, continue propagating the context change by scanning
     // the child subtree.