Use Object Spread Syntax (#7777)

* Use Object Spread Syntax

* Nits

codemods/babel-plugin-codemod-object-assign-to-object-spread/src/index.js

@@ -0,0 +1,35 @@
+import syntaxObjectRestSpread from "@babel/plugin-syntax-object-rest-spread";
+
+export default function({ types: t }) {
+  return {
+    inherits: syntaxObjectRestSpread,
+
+    visitor: {
+      CallExpression(path) {
+        if (!path.get("callee").matchesPattern("Object.assign")) return;
+
+        const args = path.get("arguments");
+        if (args.length === 0) return;
+
+        const [objPath] = args;
+        if (!objPath.isObjectExpression()) return;
+
+        const obj = objPath.node;
+        const { properties } = obj;
+
+        for (let i = 1; i < args.length; i++) {
+          const arg = args[i];
+          const { node } = arg;
+
+          if (arg.isObjectExpression()) {
+            properties.push(...node.properties);
+          } else {
+            properties.push(t.spreadElement(node));
+          }
+        }
+
+        path.replaceWith(obj);
+      },
+    },
+  };
+}