Fix asset size comparison for multi build stats (#3514)

If create-react-app project is ejected and webpack configuration is
modified to multi build setup FileSizeReporter would fail.

In those situations `webpackStats` parameter would contain stats array
for each build. This fix will try to access stats and then falls back
to using plaing webpackStats object.

packages/react-dev-utils/FileSizeReporter.js

@@ -25,21 +25,29 @@ function printFileSizesAfterBuild(
 ) {
   var root = previousSizeMap.root;
   var sizes = previousSizeMap.sizes;
-  var assets = webpackStats
-    .toJson()
-    .assets.filter(asset => /\.(js|css)$/.test(asset.name))
-    .map(asset => {
-      var fileContents = fs.readFileSync(path.join(root, asset.name));
-      var size = gzipSize(fileContents);
-      var previousSize = sizes[removeFileNameHash(root, asset.name)];
-      var difference = getDifferenceLabel(size, previousSize);
-      return {
-        folder: path.join(path.basename(buildFolder), path.dirname(asset.name)),
-        name: path.basename(asset.name),
-        size: size,
-        sizeLabel: filesize(size) + (difference ? ' (' + difference + ')' : ''),
-      };
-    });
+  var assets = (webpackStats.stats || [webpackStats])
+    .map(stats =>
+      stats
+        .toJson()
+        .assets.filter(asset => /\.(js|css)$/.test(asset.name))
+        .map(asset => {
+          var fileContents = fs.readFileSync(path.join(root, asset.name));
+          var size = gzipSize(fileContents);
+          var previousSize = sizes[removeFileNameHash(root, asset.name)];
+          var difference = getDifferenceLabel(size, previousSize);
+          return {
+            folder: path.join(
+              path.basename(buildFolder),
+              path.dirname(asset.name)
+            ),
+            name: path.basename(asset.name),
+            size: size,
+            sizeLabel:
+              filesize(size) + (difference ? ' (' + difference + ')' : '')
+          };
+        })
+    )
+    .reduce((single, all) => all.concat(single), []);
   assets.sort((a, b) => b.size - a.size);
   var longestSizeLabelLength = Math.max.apply(
     null,