Add forked copies of reconciler modules

All changes in this commit were generated by the following commands.

Copy each module that ends with `.old` to a new file that ends
with `.new`:

```sh
for f in packages/react-reconciler/src/*.old.js; do cp "$f" "$(echo "$f" | sed s/\.old/\.new/)"; done
```

Then transform the internal imports:

```sh
grep -rl --include="*.new.js" '.old' packages/react-reconciler/src/| xargs sed -i '' "s/\.old\'/\.new\'/g"
```

packages/react-reconciler/src/ReactFiberContext.new.js

@@ -0,0 +1,338 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * 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 {Fiber} from './ReactInternalTypes';
+import type {StackCursor} from './ReactFiberStack.new';
+
+import {isFiberMounted} from './ReactFiberTreeReflection';
+import {disableLegacyContext} from 'shared/ReactFeatureFlags';
+import {ClassComponent, HostRoot} from './ReactWorkTags';
+import getComponentName from 'shared/getComponentName';
+import invariant from 'shared/invariant';
+import checkPropTypes from 'shared/checkPropTypes';
+
+import {createCursor, push, pop} from './ReactFiberStack.new';
+
+let warnedAboutMissingGetChildContext;
+
+if (__DEV__) {
+  warnedAboutMissingGetChildContext = {};
+}
+
+export const emptyContextObject = {};
+if (__DEV__) {
+  Object.freeze(emptyContextObject);
+}
+
+// A cursor to the current merged context object on the stack.
+const contextStackCursor: StackCursor<Object> = createCursor(
+  emptyContextObject,
+);
+// A cursor to a boolean indicating whether the context has changed.
+const didPerformWorkStackCursor: StackCursor<boolean> = createCursor(false);
+// Keep track of the previous context object that was on the stack.
+// We use this to get access to the parent context after we have already
+// pushed the next context provider, and now need to merge their contexts.
+let previousContext: Object = emptyContextObject;
+
+function getUnmaskedContext(
+  workInProgress: Fiber,
+  Component: Function,
+  didPushOwnContextIfProvider: boolean,
+): Object {
+  if (disableLegacyContext) {
+    return emptyContextObject;
+  } else {
+    if (didPushOwnContextIfProvider && isContextProvider(Component)) {
+      // If the fiber is a context provider itself, when we read its context
+      // we may have already pushed its own child context on the stack. A context
+      // provider should not "see" its own child context. Therefore we read the
+      // previous (parent) context instead for a context provider.
+      return previousContext;
+    }
+    return contextStackCursor.current;
+  }
+}
+
+function cacheContext(
+  workInProgress: Fiber,
+  unmaskedContext: Object,
+  maskedContext: Object,
+): void {
+  if (disableLegacyContext) {
+    return;
+  } else {
+    const instance = workInProgress.stateNode;
+    instance.__reactInternalMemoizedUnmaskedChildContext = unmaskedContext;
+    instance.__reactInternalMemoizedMaskedChildContext = maskedContext;
+  }
+}
+
+function getMaskedContext(
+  workInProgress: Fiber,
+  unmaskedContext: Object,
+): Object {
+  if (disableLegacyContext) {
+    return emptyContextObject;
+  } else {
+    const type = workInProgress.type;
+    const contextTypes = type.contextTypes;
+    if (!contextTypes) {
+      return emptyContextObject;
+    }
+
+    // Avoid recreating masked context unless unmasked context has changed.
+    // Failing to do this will result in unnecessary calls to componentWillReceiveProps.
+    // This may trigger infinite loops if componentWillReceiveProps calls setState.
+    const instance = workInProgress.stateNode;
+    if (
+      instance &&
+      instance.__reactInternalMemoizedUnmaskedChildContext === unmaskedContext
+    ) {
+      return instance.__reactInternalMemoizedMaskedChildContext;
+    }
+
+    const context = {};
+    for (const key in contextTypes) {
+      context[key] = unmaskedContext[key];
+    }
+
+    if (__DEV__) {
+      const name = getComponentName(type) || 'Unknown';
+      checkPropTypes(contextTypes, context, 'context', name);
+    }
+
+    // Cache unmasked context so we can avoid recreating masked context unless necessary.
+    // Context is created before the class component is instantiated so check for instance.
+    if (instance) {
+      cacheContext(workInProgress, unmaskedContext, context);
+    }
+
+    return context;
+  }
+}
+
+function hasContextChanged(): boolean {
+  if (disableLegacyContext) {
+    return false;
+  } else {
+    return didPerformWorkStackCursor.current;
+  }
+}
+
+function isContextProvider(type: Function): boolean {
+  if (disableLegacyContext) {
+    return false;
+  } else {
+    const childContextTypes = type.childContextTypes;
+    return childContextTypes !== null && childContextTypes !== undefined;
+  }
+}
+
+function popContext(fiber: Fiber): void {
+  if (disableLegacyContext) {
+    return;
+  } else {
+    pop(didPerformWorkStackCursor, fiber);
+    pop(contextStackCursor, fiber);
+  }
+}
+
+function popTopLevelContextObject(fiber: Fiber): void {
+  if (disableLegacyContext) {
+    return;
+  } else {
+    pop(didPerformWorkStackCursor, fiber);
+    pop(contextStackCursor, fiber);
+  }
+}
+
+function pushTopLevelContextObject(
+  fiber: Fiber,
+  context: Object,
+  didChange: boolean,
+): void {
+  if (disableLegacyContext) {
+    return;
+  } else {
+    invariant(
+      contextStackCursor.current === emptyContextObject,
+      'Unexpected context found on stack. ' +
+        'This error is likely caused by a bug in React. Please file an issue.',
+    );
+
+    push(contextStackCursor, context, fiber);
+    push(didPerformWorkStackCursor, didChange, fiber);
+  }
+}
+
+function processChildContext(
+  fiber: Fiber,
+  type: any,
+  parentContext: Object,
+): Object {
+  if (disableLegacyContext) {
+    return parentContext;
+  } else {
+    const instance = fiber.stateNode;
+    const childContextTypes = type.childContextTypes;
+
+    // TODO (bvaughn) Replace this behavior with an invariant() in the future.
+    // It has only been added in Fiber to match the (unintentional) behavior in Stack.
+    if (typeof instance.getChildContext !== 'function') {
+      if (__DEV__) {
+        const componentName = getComponentName(type) || 'Unknown';
+
+        if (!warnedAboutMissingGetChildContext[componentName]) {
+          warnedAboutMissingGetChildContext[componentName] = true;
+          console.error(
+            '%s.childContextTypes is specified but there is no getChildContext() method ' +
+              'on the instance. You can either define getChildContext() on %s or remove ' +
+              'childContextTypes from it.',
+            componentName,
+            componentName,
+          );
+        }
+      }
+      return parentContext;
+    }
+
+    const childContext = instance.getChildContext();
+    for (const contextKey in childContext) {
+      invariant(
+        contextKey in childContextTypes,
+        '%s.getChildContext(): key "%s" is not defined in childContextTypes.',
+        getComponentName(type) || 'Unknown',
+        contextKey,
+      );
+    }
+    if (__DEV__) {
+      const name = getComponentName(type) || 'Unknown';
+      checkPropTypes(childContextTypes, childContext, 'child context', name);
+    }
+
+    return {...parentContext, ...childContext};
+  }
+}
+
+function pushContextProvider(workInProgress: Fiber): boolean {
+  if (disableLegacyContext) {
+    return false;
+  } else {
+    const instance = workInProgress.stateNode;
+    // We push the context as early as possible to ensure stack integrity.
+    // If the instance does not exist yet, we will push null at first,
+    // and replace it on the stack later when invalidating the context.
+    const memoizedMergedChildContext =
+      (instance && instance.__reactInternalMemoizedMergedChildContext) ||
+      emptyContextObject;
+
+    // Remember the parent context so we can merge with it later.
+    // Inherit the parent's did-perform-work value to avoid inadvertently blocking updates.
+    previousContext = contextStackCursor.current;
+    push(contextStackCursor, memoizedMergedChildContext, workInProgress);
+    push(
+      didPerformWorkStackCursor,
+      didPerformWorkStackCursor.current,
+      workInProgress,
+    );
+
+    return true;
+  }
+}
+
+function invalidateContextProvider(
+  workInProgress: Fiber,
+  type: any,
+  didChange: boolean,
+): void {
+  if (disableLegacyContext) {
+    return;
+  } else {
+    const instance = workInProgress.stateNode;
+    invariant(
+      instance,
+      'Expected to have an instance by this point. ' +
+        'This error is likely caused by a bug in React. Please file an issue.',
+    );
+
+    if (didChange) {
+      // Merge parent and own context.
+      // Skip this if we're not updating due to sCU.
+      // This avoids unnecessarily recomputing memoized values.
+      const mergedContext = processChildContext(
+        workInProgress,
+        type,
+        previousContext,
+      );
+      instance.__reactInternalMemoizedMergedChildContext = mergedContext;
+
+      // Replace the old (or empty) context with the new one.
+      // It is important to unwind the context in the reverse order.
+      pop(didPerformWorkStackCursor, workInProgress);
+      pop(contextStackCursor, workInProgress);
+      // Now push the new context and mark that it has changed.
+      push(contextStackCursor, mergedContext, workInProgress);
+      push(didPerformWorkStackCursor, didChange, workInProgress);
+    } else {
+      pop(didPerformWorkStackCursor, workInProgress);
+      push(didPerformWorkStackCursor, didChange, workInProgress);
+    }
+  }
+}
+
+function findCurrentUnmaskedContext(fiber: Fiber): Object {
+  if (disableLegacyContext) {
+    return emptyContextObject;
+  } else {
+    // Currently this is only used with renderSubtreeIntoContainer; not sure if it
+    // makes sense elsewhere
+    invariant(
+      isFiberMounted(fiber) && fiber.tag === ClassComponent,
+      'Expected subtree parent to be a mounted class component. ' +
+        'This error is likely caused by a bug in React. Please file an issue.',
+    );
+
+    let node = fiber;
+    do {
+      switch (node.tag) {
+        case HostRoot:
+          return node.stateNode.context;
+        case ClassComponent: {
+          const Component = node.type;
+          if (isContextProvider(Component)) {
+            return node.stateNode.__reactInternalMemoizedMergedChildContext;
+          }
+          break;
+        }
+      }
+      node = node.return;
+    } while (node !== null);
+    invariant(
+      false,
+      'Found unexpected detached subtree parent. ' +
+        'This error is likely caused by a bug in React. Please file an issue.',
+    );
+  }
+}
+
+export {
+  getUnmaskedContext,
+  cacheContext,
+  getMaskedContext,
+  hasContextChanged,
+  popContext,
+  popTopLevelContextObject,
+  pushTopLevelContextObject,
+  processChildContext,
+  isContextProvider,
+  pushContextProvider,
+  invalidateContextProvider,
+  findCurrentUnmaskedContext,
+};