Codemod Object.assign with object literal first argument to object spread in Xplat

Summary:
Codemod `Object.assign` with object literal first argument (e.g. `Object.assign({}, foo)`) to object spread.

This adds several suppressions for exposed errors. The codemod produces errors as `Object.assign` is more unsafe than object spread. For example, `Object.assign` doesn't handle indexers, nor does it handle inexact objects properly, and when the (currently unsealed) empty object is supplied as the first argument, it also leads to unsafe behaviour:
https://flow.org/try/#0FAehAIGJwSQOwCYFMAeSBOBnYyDGAbAQ3SXADdjwBbQgBwC5wBvAbUwBd0BLOAcwF1GHbnwC+AbmDAAFAHkARgCskudgDpCmTF15xpTQowCMogDTU6ASkbyA9rfxJCcS+PBhwAfm8ymwcAGG4Eam-gFqETS0oaKu7hAAyrQkhAjgGOi2WOCa6Si0KuxICFIe0PCohKo5AGZF6HlV7DgqRCTklDyVqowGQpw8vOYRahJSCsqqGlo6en3gAEQAFlwL5vLGZuBdKE1xHjtN4Li2AK74abZkGVzI4AAG8vfgUvphOYzLq6EB4BvBP3CEUOqhi+0SyScaQyWUwOThqAKqmKpQg0AAqnBME5HGkalwsOwcuheDIJoVptpdPotvMTNZmDV7Iw4KcqPIMLE3B5vN4gA

The codemod is safe to do, despite some Flow errors, as `Object.assign` and object spread are equivalent at runtime, with the exception that `Object.assign` triggers setters on the target object. However the codemod [does not run](https://github.com/eslint/eslint/blob/938dbdd6c310784cc8a7329efaeb0e34321b9e1f/lib/rules/prefer-object-spread.js#L283-L285) if the first argument (object literal) has getters/setters, so we are fine.

```
ag -l 'Object.assign\(' | xargs ag -l 'flow' | xargs js1 lint --rule '{"prefer-object-spread":2}' --fix
```
Some manual fixes
```
arc f
```

Reviewed By: SamChou19815

Differential Revision: D36023786

fbshipit-source-id: b682562e670410acf4175ba59ab285c7bdcfe052

Libraries/Components/StatusBar/StatusBar.js

@@ -112,14 +112,17 @@ function mergePropsStack(
   propsStack: Array<Object>,
   defaultValues: Object,
 ): Object {
-  return propsStack.reduce((prev, cur) => {
-    for (const prop in cur) {
-      if (cur[prop] != null) {
-        prev[prop] = cur[prop];
+  return propsStack.reduce(
+    (prev, cur) => {
+      for (const prop in cur) {
+        if (cur[prop] != null) {
+          prev[prop] = cur[prop];
+        }
       }
-    }
-    return prev;
-  }, Object.assign({}, defaultValues));
+      return prev;
+    },
+    {...defaultValues},
+  );
 }
 
 /**

Libraries/Inspector/ElementBox.js

@@ -102,7 +102,7 @@ type Style = {
  * @return a modified copy
  */
 function resolveRelativeSizes(style: $ReadOnly<Style>): Style {
-  let resolvedStyle = Object.assign({}, style);
+  let resolvedStyle = {...style};
   resolveSizeInPlace(resolvedStyle, 'top', 'height');
   resolveSizeInPlace(resolvedStyle, 'right', 'width');
   resolveSizeInPlace(resolvedStyle, 'bottom', 'height');

Libraries/LogBox/Data/__tests__/LogBoxData-test.js

@@ -70,24 +70,19 @@ const addLogs = (logs, options) => {
 
 const addSoftErrors = (errors, options) => {
   errors.forEach(error => {
-    LogBoxData.addException(
-      Object.assign(
-        {},
-        {
-          message: '',
-          isComponentError: false,
-          originalMessage: '',
-          name: 'console.error',
-          componentStack: '',
-          stack: [],
-          id: 0,
-          isFatal: false,
-        },
-        typeof error === 'string'
-          ? {message: error, originalMessage: error}
-          : error,
-      ),
-    );
+    LogBoxData.addException({
+      message: '',
+      isComponentError: false,
+      originalMessage: '',
+      name: 'console.error',
+      componentStack: '',
+      stack: [],
+      id: 0,
+      isFatal: false,
+      ...(typeof error === 'string'
+        ? {message: error, originalMessage: error}
+        : error),
+    });
     if (options == null || options.flush !== false) {
       jest.runOnlyPendingTimers();
     }
@@ -96,24 +91,19 @@ const addSoftErrors = (errors, options) => {
 
 const addFatalErrors = (errors, options) => {
   errors.forEach(error => {
-    LogBoxData.addException(
-      Object.assign(
-        {},
-        {
-          message: '',
-          isComponentError: false,
-          originalMessage: '',
-          name: 'console.error',
-          componentStack: '',
-          stack: [],
-          id: 0,
-          isFatal: true,
-        },
-        typeof error === 'string'
-          ? {message: error, originalMessage: error}
-          : error,
-      ),
-    );
+    LogBoxData.addException({
+      message: '',
+      isComponentError: false,
+      originalMessage: '',
+      name: 'console.error',
+      componentStack: '',
+      stack: [],
+      id: 0,
+      isFatal: true,
+      ...(typeof error === 'string'
+        ? {message: error, originalMessage: error}
+        : error),
+    });
     if (options == null || options.flush !== false) {
       // Errors include two timers, the second is for optimistic symbolication.
       jest.runOnlyPendingTimers();

Libraries/Utilities/truncate.js

@@ -29,7 +29,7 @@ const truncate = function (
   maxChars: number,
   options?: truncateOptions,
 ): ?string {
-  options = Object.assign({}, defaultOptions, options);
+  options = {...defaultOptions, ...options};
   if (
     str &&
     str.length &&