Re-enable Flow for ReactFiber and fix Flow issues

.flowconfig

@@ -21,6 +21,9 @@
 ./node_modules/fbjs/flow/lib/dev.js
 ./scripts/flow
 
+[lints]
+untyped-type-import=error
+
 [options]
 esproposal.class_static_fields=enable
 esproposal.class_instance_fields=enable

packages/react-dom/src/events/ReactDOMEventListener.js

@@ -54,7 +54,7 @@ function getTopLevelCallbackBookKeeping(
 ): {
   topLevelType: ?DOMTopLevelEventType,
   nativeEvent: ?AnyNativeEvent,
-  targetInst: Fiber,
+  targetInst: Fiber | null,
   ancestors: Array<Fiber>,
 } {
   if (callbackBookkeepingPool.length) {

packages/react-reconciler/src/ReactCapturedValue.js

@@ -29,7 +29,7 @@ export type CapturedError = {
 
 export function createCapturedValue<T>(
   value: T,
-  source: Fiber | null,
+  source: Fiber,
 ): CapturedValue<T> {
   // If the value is an error, call this function immediately after it is thrown
   // so the stack is accurate.

packages/react-reconciler/src/ReactFiber.js

@@ -4,10 +4,11 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
+ * @flow
  */
 
 import type {ReactElement, Source} from 'shared/ReactElementType';
-import type {ReactPortal, RefObject} from 'shared/ReactTypes';
+import type {ReactFragment, ReactPortal, RefObject} from 'shared/ReactTypes';
 import type {TypeOfWork} from 'shared/ReactTypeOfWork';
 import type {TypeOfMode} from './ReactTypeOfMode';
 import type {TypeOfSideEffect} from 'shared/ReactTypeOfSideEffect';
@@ -316,7 +317,7 @@ export function createWorkInProgress(
   return workInProgress;
 }
 
-export function createHostRootFiber(isAsync): Fiber {
+export function createHostRootFiber(isAsync: boolean): Fiber {
   const mode = isAsync ? AsyncMode | StrictMode : NoContext;
   return createFiber(HostRoot, null, null, mode);
 }
@@ -366,42 +367,9 @@ export function createFiberFromElement(
         // mode compatible.
         mode |= StrictMode;
         break;
-      default: {
-        if (typeof type === 'object' && type !== null) {
-          switch (type.$$typeof) {
-            case REACT_PROVIDER_TYPE:
-              fiberTag = ContextProvider;
-              break;
-            case REACT_CONTEXT_TYPE:
-              // This is a consumer
-              fiberTag = ContextConsumer;
-              break;
-            case REACT_FORWARD_REF_TYPE:
-              fiberTag = ForwardRef;
-              break;
-            default:
-              if (typeof type.tag === 'number') {
-                // Currently assumed to be a continuation and therefore is a
-                // fiber already.
-                // TODO: The yield system is currently broken for updates in
-                // some cases. The reified yield stores a fiber, but we don't
-                // know which fiber that is; the current or a workInProgress?
-                // When the continuation gets rendered here we don't know if we
-                // can reuse that fiber or if we need to clone it. There is
-                // probably a clever way to restructure this.
-                fiber = ((type: any): Fiber);
-                fiber.pendingProps = pendingProps;
-                fiber.expirationTime = expirationTime;
-                return fiber;
-              } else {
-                throwOnInvalidElementType(type, owner);
-              }
-              break;
-          }
-        } else {
-          throwOnInvalidElementType(type, owner);
-        }
-      }
+      default:
+        fiberTag = getFiberTagFromObjectType(type, owner);
+        break;
     }
   }
 
@@ -417,33 +385,47 @@ export function createFiberFromElement(
   return fiber;
 }
 
-function throwOnInvalidElementType(type, owner) {
-  let info = '';
-  if (__DEV__) {
-    if (
-      type === undefined ||
-      (typeof type === 'object' &&
-        type !== null &&
-        Object.keys(type).length === 0)
-    ) {
-      info +=
-        ' You likely forgot to export your component from the file ' +
-        "it's defined in, or you might have mixed up default and " +
-        'named imports.';
-    }
-    const ownerName = owner ? getComponentName(owner) : null;
-    if (ownerName) {
-      info += '\n\nCheck the render method of `' + ownerName + '`.';
+function getFiberTagFromObjectType(type, owner): TypeOfWork {
+  const $$typeof =
+    typeof type === 'object' && type !== null ? type.$$typeof : null;
+
+  switch ($$typeof) {
+    case REACT_PROVIDER_TYPE:
+      return ContextProvider;
+    case REACT_CONTEXT_TYPE:
+      // This is a consumer
+      return ContextConsumer;
+    case REACT_FORWARD_REF_TYPE:
+      return ForwardRef;
+    default: {
+      let info = '';
+      if (__DEV__) {
+        if (
+          type === undefined ||
+          (typeof type === 'object' &&
+            type !== null &&
+            Object.keys(type).length === 0)
+        ) {
+          info +=
+            ' You likely forgot to export your component from the file ' +
+            "it's defined in, or you might have mixed up default and " +
+            'named imports.';
+        }
+        const ownerName = owner ? getComponentName(owner) : null;
+        if (ownerName) {
+          info += '\n\nCheck the render method of `' + ownerName + '`.';
+        }
+      }
+      invariant(
+        false,
+        'Element type is invalid: expected a string (for built-in ' +
+          'components) or a class/function (for composite components) ' +
+          'but got: %s.%s',
+        type == null ? type : typeof type,
+        info,
+      );
     }
   }
-  invariant(
-    false,
-    'Element type is invalid: expected a string (for built-in ' +
-      'components) or a class/function (for composite components) ' +
-      'but got: %s.%s',
-    type == null ? type : typeof type,
-    info,
-  );
 }
 
 export function createFiberFromFragment(

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -9,7 +9,7 @@
 
 import type {HostConfig} from 'react-reconciler';
 import type {Fiber} from './ReactFiber';
-import type {FiberRoot} from './ReactFiber';
+import type {FiberRoot} from './ReactFiberRoot';
 import type {ExpirationTime} from './ReactFiberExpirationTime';
 import type {CapturedValue, CapturedError} from './ReactCapturedValue';
 
@@ -59,7 +59,7 @@ if (__DEV__) {
 export function logError(boundary: Fiber, errorInfo: CapturedValue<mixed>) {
   const source = errorInfo.source;
   let stack = errorInfo.stack;
-  if (stack === null) {
+  if (stack === null && source !== null) {
     stack = getStackAddendumByWorkInProgressFiber(source);
   }
 
@@ -94,7 +94,7 @@ export function logError(boundary: Fiber, errorInfo: CapturedValue<mixed>) {
 
 export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
   config: HostConfig<T, P, I, TI, HI, PI, C, CC, CX, PL>,
-  captureError: (failedFiber: Fiber, error: mixed) => Fiber | null,
+  captureError: (failedFiber: Fiber, error: mixed) => void,
   scheduleWork: (
     fiber: Fiber,
     startTime: ExpirationTime,

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -321,10 +321,20 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
       }
 
       // Restore the original state of the work-in-progress
+      if (stashedWorkInProgressProperties === null) {
+        // This should never happen. Don't throw because this code is DEV-only.
+        warning(
+          false,
+          'Could not replay rendering after an error. This is likely a bug in React. ' +
+            'Please file an issue.',
+        );
+        return;
+      }
       assignFiberPropertiesInDEV(
         failedUnitOfWork,
         stashedWorkInProgressProperties,
       );
+
       switch (failedUnitOfWork.tag) {
         case HostRoot:
           popHostContainer(failedUnitOfWork);
@@ -1073,43 +1083,52 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
           // This is a fatal error.
           didFatal = true;
           onUncaughtError(thrownValue);
-          break;
-        }
+        } else {
+          if (__DEV__) {
+            // Reset global debug state
+            // We assume this is defined in DEV
+            (resetCurrentlyProcessingQueue: any)();
+          }
 
-        if (__DEV__) {
-          // Reset global debug state
-          // We assume this is defined in DEV
-          (resetCurrentlyProcessingQueue: any)();
-        }
+          const failedUnitOfWork: Fiber = nextUnitOfWork;
+          if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
+            replayUnitOfWork(failedUnitOfWork, thrownValue, isAsync);
+          }
 
-        if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
-          const failedUnitOfWork = nextUnitOfWork;
-          replayUnitOfWork(failedUnitOfWork, thrownValue, isAsync);
-        }
+          // TODO: we already know this isn't true in some cases.
+          // At least this shows a nicer error message until we figure out the cause.
+          // https://github.com/facebook/react/issues/12449#issuecomment-386727431
+          invariant(
+            nextUnitOfWork !== null,
+            'Failed to replay rendering after an error. This ' +
+              'is likely caused by a bug in React. Please file an issue ' +
+              'with a reproducing case to help us find it.',
+          );
 
-        const sourceFiber: Fiber = nextUnitOfWork;
-        let returnFiber = sourceFiber.return;
-        if (returnFiber === null) {
-          // This is the root. The root could capture its own errors. However,
-          // we don't know if it errors before or after we pushed the host
-          // context. This information is needed to avoid a stack mismatch.
-          // Because we're not sure, treat this as a fatal error. We could track
-          // which phase it fails in, but doesn't seem worth it. At least
-          // for now.
-          didFatal = true;
-          onUncaughtError(thrownValue);
-          break;
+          const sourceFiber: Fiber = nextUnitOfWork;
+          let returnFiber = sourceFiber.return;
+          if (returnFiber === null) {
+            // This is the root. The root could capture its own errors. However,
+            // we don't know if it errors before or after we pushed the host
+            // context. This information is needed to avoid a stack mismatch.
+            // Because we're not sure, treat this as a fatal error. We could track
+            // which phase it fails in, but doesn't seem worth it. At least
+            // for now.
+            didFatal = true;
+            onUncaughtError(thrownValue);
+            break;
+          }
+          throwException(
+            root,
+            returnFiber,
+            sourceFiber,
+            thrownValue,
+            nextRenderIsExpired,
+            nextRenderExpirationTime,
+            mostRecentCurrentTimeMs,
+          );
+          nextUnitOfWork = completeUnitOfWork(sourceFiber);
         }
-        throwException(
-          root,
-          returnFiber,
-          sourceFiber,
-          thrownValue,
-          nextRenderIsExpired,
-          nextRenderExpirationTime,
-          mostRecentCurrentTimeMs,
-        );
-        nextUnitOfWork = completeUnitOfWork(sourceFiber);
       }
       break;
     } while (true);

packages/react-reconciler/src/ReactStrictModeWarnings.js

@@ -107,15 +107,15 @@ if (__DEV__) {
     pendingUnsafeLifecycleWarnings = new Map();
   };
 
-  const getStrictRoot = (fiber: Fiber): Fiber => {
+  const findStrictRoot = (fiber: Fiber): Fiber | null => {
     let maybeStrictRoot = null;
 
-    while (fiber !== null) {
-      if (fiber.mode & StrictMode) {
-        maybeStrictRoot = fiber;
+    let node = fiber;
+    while (node !== null) {
+      if (node.mode & StrictMode) {
+        maybeStrictRoot = node;
       }
-
-      fiber = fiber.return;
+      node = node.return;
     }
 
     return maybeStrictRoot;
@@ -225,7 +225,15 @@ if (__DEV__) {
     fiber: Fiber,
     instance: any,
   ) => {
-    const strictRoot = getStrictRoot(fiber);
+    const strictRoot = findStrictRoot(fiber);
+    if (strictRoot === null) {
+      warning(
+        false,
+        'Expected to find a StrictMode component in a strict mode tree. ' +
+          'This error is likely caused by a bug in React. Please file an issue.',
+      );
+      return;
+    }
 
     // Dedup strategy: Warn once per component.
     // This is difficult to track any other way since component names