[flow] enable exact_empty_objects (#25973)

This enables the "exact_empty_objects" setting for Flow which makes
empty objects exact instead of building up the type as properties are
added in code below. This is in preparation to Flow 191 which makes this
the default and removes the config.

More about the change in the Flow blog
[here](https://medium.com/flow-type/improved-handling-of-the-empty-object-in-flow-ead91887e40c).

Author: kassens

packages/react-client/src/ReactFlightClient.js

@@ -393,7 +393,9 @@ function createElement(
     // We don't really need to add any of these but keeping them for good measure.
     // Unfortunately, _store is enumerable in jest matchers so for equality to
     // work, I need to keep it or make _store non-enumerable in the other file.
-    element._store = {};
+    element._store = ({}: {
+      validated?: boolean,
+    });
     Object.defineProperty(element._store, 'validated', {
       configurable: false,
       enumerable: false,

packages/react-debug-tools/src/ReactDebugHooks.js

@@ -524,7 +524,7 @@ function buildTree(
   readHookLog: Array<HookLogEntry>,
   includeHooksSource: boolean,
 ): HooksTree {
-  const rootChildren = [];
+  const rootChildren: Array<HooksNode> = [];
   let prevStack = null;
   let levelChildren = rootChildren;
   let nativeHookID = 0;
@@ -557,7 +557,7 @@ function buildTree(
       // The remaining part of the new stack are custom hooks. Push them
       // to the tree.
       for (let j = stack.length - commonSteps - 1; j >= 1; j--) {
-        const children = [];
+        const children: Array<HooksNode> = [];
         const stackFrame = stack[j];
         const levelChild: HooksNode = {
           id: null,

packages/react-devtools-shared/src/backend/NativeStyleEditor/setupNativeStyleEditor.js

@@ -170,7 +170,7 @@ function measureStyle(
 }
 
 function shallowClone(object: Object): Object {
-  const cloned = {};
+  const cloned: {[string]: $FlowFixMe} = {};
   for (const n in object) {
     cloned[n] = object[n];
   }

packages/react-devtools-shared/src/backend/console.js

@@ -79,7 +79,7 @@ const injectedRenderers: Map<
 > = new Map();
 
 let targetConsole: Object = console;
-let targetConsoleMethods = {};
+let targetConsoleMethods: {[string]: $FlowFixMe} = {};
 for (const method in console) {
   targetConsoleMethods[method] = console[method];
 }
@@ -97,7 +97,7 @@ export function dangerous_setTargetConsoleForTesting(
 ): void {
   targetConsole = targetConsoleForTesting;
 
-  targetConsoleMethods = {};
+  targetConsoleMethods = ({}: {[string]: $FlowFixMe});
   for (const method in targetConsole) {
     targetConsoleMethods[method] = console[method];
   }
@@ -179,7 +179,7 @@ export function patch({
       return;
     }
 
-    const originalConsoleMethods = {};
+    const originalConsoleMethods: {[string]: $FlowFixMe} = {};
 
     unpatchFn = () => {
       for (const method in originalConsoleMethods) {
@@ -318,7 +318,7 @@ export function patchForStrictMode() {
       return;
     }
 
-    const originalConsoleMethods = {};
+    const originalConsoleMethods: {[string]: $FlowFixMe} = {};
 
     unpatchForStrictModeFn = () => {
       for (const method in originalConsoleMethods) {

packages/react-devtools-shared/src/backend/legacy/renderer.js

@@ -785,7 +785,7 @@ export function attach(
 
       let owner = element._owner;
       if (owner) {
-        owners = [];
+        owners = ([]: Array<SerializedElement>);
         while (owner != null) {
           owners.push({
             displayName: getData(owner).displayName || 'Unknown',
@@ -807,8 +807,8 @@ export function attach(
     }
 
     // Not implemented
-    const errors = [];
-    const warnings = [];
+    const errors: Array<[string, number]> = [];
+    const warnings: Array<[string, number]> = [];
 
     return {
       id,

packages/react-devtools-shared/src/backend/legacy/utils.js

@@ -22,7 +22,7 @@ export function decorateMany(
   source: Object,
   fns: {[attr: string]: Function, ...},
 ): Object {
-  const olds = {};
+  const olds: {[string]: $FlowFixMe} = {};
   for (const name in fns) {
     olds[name] = decorate(source, name, fns[name]);
   }

packages/react-devtools-shared/src/backend/profilingHooks.js

@@ -52,7 +52,7 @@ let supportsUserTiming =
 let supportsUserTimingV3 = false;
 if (supportsUserTiming) {
   const CHECK_V3_MARK = '__v3';
-  const markOptions = {};
+  const markOptions = ({}: {startTime?: number});
   Object.defineProperty(markOptions, 'startTime', {
     get: function() {
       supportsUserTimingV3 = true;

packages/react-devtools-shared/src/backend/renderer.js

@@ -3252,7 +3252,7 @@ export function attach(
 
     let owners = null;
     if (_debugOwner) {
-      owners = [];
+      owners = ([]: Array<SerializedElement>);
       let owner: null | Fiber = _debugOwner;
       while (owner !== null) {
         owners.push(fiberToSerializedElement(owner));
@@ -3265,7 +3265,7 @@ export function attach(
 
     let hooks = null;
     if (usesHooks) {
-      const originalConsoleMethods = {};
+      const originalConsoleMethods: {[string]: $FlowFixMe} = {};
 
       // Temporarily disable all console logging before re-running the hook.
       for (const method in console) {

packages/react-devtools-shared/src/backend/utils.js

@@ -20,8 +20,8 @@ export function cleanForBridge(
   path?: Array<string | number> = [],
 ): DehydratedData | null {
   if (data !== null) {
-    const cleanedPaths = [];
-    const unserializablePaths = [];
+    const cleanedPaths: Array<Array<string | number>> = [];
+    const unserializablePaths: Array<Array<string | number>> = [];
     const cleanedData = dehydrate(
       data,
       cleanedPaths,

packages/react-devtools-shared/src/devtools/utils.js

@@ -198,7 +198,7 @@ const STACK_SOURCE_LOCATION = /([^\s]+) \((.+):(.+):(.+)\)/;
 export function stackToComponentSources(
   stack: string,
 ): Array<[string, ?Stack]> {
-  const out = [];
+  const out: Array<[string, ?Stack]> = [];
   stack
     .split(STACK_DELIMETER)
     .slice(1)