version detection: detect: only warn once

Closes jsx-eslint#2276.

Author: ljharb

lib/util/version.js

@@ -9,15 +9,20 @@ const error = require('./error');
 
 let warnedForMissingVersion = false;
 
+function resetWarningFlag() {
+  warnedForMissingVersion = false;
+}
+
 function detectReactVersion() {
   try {
     const reactPath = resolve.sync('react', {basedir: process.cwd()});
     const react = require(reactPath);
     return react.version;
   } catch (e) {
-    if (e.code === 'MODULE_NOT_FOUND') {
+    if (!warnedForMissingVersion && e.code === 'MODULE_NOT_FOUND') {
       error('Warning: React version was set to "detect" in eslint-plugin-react settings, ' +
         'but the "react" package is not installed. Assuming latest React version for linting.');
+      warnedForMissingVersion = true;
       return '999.999.999';
     }
     throw e;
@@ -107,5 +112,6 @@ function testFlowVersion(context, methodVer) {
 
 module.exports = {
   testReactVersion,
-  testFlowVersion
+  testFlowVersion,
+  resetWarningFlag
 };

tests/util/version.js

@@ -15,6 +15,7 @@ describe('Version', () => {
     process.chdir(base);
     sinon.stub(console, 'error');
     expectedErrorArgs = [];
+    versionUtil.resetWarningFlag();
   });
 
   afterEach(() => {