Don't reset the current fiber during reconciliation

It should only be reset when we stop doing work.
Otherwise, any warnings after the reset will lose the component stack.

The reset in getMaskedContext() was completely unnecessary.
It is always called with the same fiber as the current work in progress.
Therefore, I add a DEV-only warning assertion to ensure we don't regress, and remove the reset.

The reset in processChildContext() is necessary because it can be called outside of reconciliation.
Unfortunately, we have to keep this hack in until we can remove unstable_renderSubtreeIntoContainer().
To work around it, I restore the previous fiber instead of resetting.

src/renderers/shared/fiber/ReactFiberContext.js

@@ -89,15 +89,20 @@ exports.getMaskedContext = function(
 
   if (__DEV__) {
     const name = getComponentName(workInProgress) || 'Unknown';
-    ReactDebugCurrentFiber.setCurrentFiber(workInProgress, null);
+    if (workInProgress !== ReactDebugCurrentFiber.current) {
+      warning(
+        false,
+        'Expected the work in progress to match the currently processed fiber. ' +
+        'This error is likely caused by a bug in React. Please file an issue.'
+      );
+    }
     checkPropTypes(
       contextTypes,
       context,
       'context',
       name,
       ReactDebugCurrentFiber.getCurrentFiberStackAddendum,
     );
-    ReactDebugCurrentFiber.resetCurrentFiber();
   }
 
   // Cache unmasked context so we can avoid recreating masked context unless necessary.
@@ -184,11 +189,16 @@ function processChildContext(
 
   let childContext;
   if (__DEV__) {
+    // TODO: we only have to store the "previous" fiber and phase and restore them
+    // because this method can be called outside of reconciliation. We can remove this
+    // when we stop supporting unstable_renderSubtreeIntoContainer.
+    const previousCurrentFiber = ReactDebugCurrentFiber.current;
+    const previousCurrentPhase = ReactDebugCurrentFiber.phase;
     ReactDebugCurrentFiber.setCurrentFiber(fiber, 'getChildContext');
     startPhaseTimer(fiber, 'getChildContext');
     childContext = instance.getChildContext();
     stopPhaseTimer();
-    ReactDebugCurrentFiber.resetCurrentFiber();
+    ReactDebugCurrentFiber.setCurrentFiber(previousCurrentFiber, previousCurrentPhase);
   } else {
     childContext = instance.getChildContext();
   }
@@ -208,6 +218,11 @@ function processChildContext(
     // assume anything about the given fiber. We won't pass it down if we aren't sure.
     // TODO: remove this hack when we delete unstable_renderSubtree in Fiber.
     const workInProgress = isReconciling ? fiber : null;
+    // TODO: we only have to store the "previous" fiber and phase and restore them
+    // because this method can be called outside of reconciliation. We can remove this
+    // when we stop supporting unstable_renderSubtreeIntoContainer.
+    const previousCurrentFiber = ReactDebugCurrentFiber.current;
+    const previousCurrentPhase = ReactDebugCurrentFiber.phase;
     ReactDebugCurrentFiber.setCurrentFiber(workInProgress, null);
     checkPropTypes(
       childContextTypes,
@@ -216,7 +231,7 @@ function processChildContext(
       name,
       ReactDebugCurrentFiber.getCurrentFiberStackAddendum,
     );
-    ReactDebugCurrentFiber.resetCurrentFiber();
+    ReactDebugCurrentFiber.setCurrentFiber(previousCurrentFiber, previousCurrentPhase);
   }
 
   return {...parentContext, ...childContext};