Try to update /tests/index.html only if it exists (guard from prod bu…

…ild failures)

index.js

@@ -21,7 +21,11 @@ module.exports = {
     let testIndexFilePath = build.directory + '/tests/index.html';
 
     let indexFile = fs.readFileSync(indexFilePath, {encoding: 'utf-8'});
-    let testIndexFile = fs.readFileSync(testIndexFilePath, {encoding: 'utf-8'});
+
+    let testIndexFile;
+    if (fs.existsSync(testIndexFilePath)) {
+      testIndexFile = fs.readFileSync(testIndexFilePath, {encoding: 'utf-8'});
+    }
 
     let files = fs.readdirSync(build.directory + '/assets');
     let totalFiles = files.length;
@@ -41,13 +45,16 @@ module.exports = {
     }
 
     let assetMapContent = null;
-    
+
     if (assetFileName) {
       assetMapContent = `"${fingerprintPrepend + 'assets/' + assetFileName}"`;
     }
-    
+
     fs.writeFileSync(indexFilePath, indexFile.replace(/__asset_map_placeholder__/, assetMapContent));
-    fs.writeFileSync(testIndexFilePath, testIndexFile.replace(/__asset_map_placeholder__/, assetMapContent));
+
+    if (testIndexFile) {
+      fs.writeFileSync(testIndexFilePath, testIndexFile.replace(/__asset_map_placeholder__/, assetMapContent));
+    }
   },
 
   contentFor(type, config) {