Pre-suppress unsafe string key access errors in xplat/js

Summary:
X-link: facebook/react-native#44221

This diff pre-suppresses errors of the following pattern, to prepare for the next Flow release.

```
declare const obj: {foo: string};
declare const key: string;
obj[key]; // error: invalid-computed-prop
```

Changelog: [Internal]

Reviewed By: alexmckenley

Differential Revision: D56477899

fbshipit-source-id: 5676b8685bd3157a519fe433cfce0fa28e003502

packages/metro-resolver/src/PackageResolve.js

@@ -31,6 +31,7 @@ export function getPackageEntryPoint(
   let main = 'index';
 
   for (const name of mainFields) {
+    // $FlowFixMe[invalid-computed-prop]
     if (typeof pkg[name] === 'string' && pkg[name].length) {
       main = pkg[name];
       break;
@@ -165,6 +166,7 @@ function matchSubpathFromMainFields(
   mainFields: $ReadOnlyArray<string>,
 ): string | false | null {
   const fieldValues = mainFields
+    // $FlowFixMe[invalid-computed-prop]
     .map(name => pkg[name])
     .filter(value => value != null && typeof value !== 'string');
 

packages/metro/src/lib/logToConsole.js

@@ -26,6 +26,7 @@ module.exports = (
   mode: 'BRIDGE' | 'NOBRIDGE',
   ...data: Array<mixed>
 ) => {
+  // $FlowFixMe[invalid-computed-prop]
   const logFunction = console[level] && level !== 'trace' ? level : 'log';
   const color =
     level === 'error'