No unneeded empty arrays in transform spread (#6763)

* No unneeded empty arrays in transform spread

Since Array.prototype.concat creates a new array from inputs, there's
no need to call it from a new empty array ([].concat()).

* [fixup] simplify detection of new array

packages/babel-plugin-transform-spread/README.md

@@ -15,7 +15,7 @@ var b = [...a, 'foo'];
 
 ```js
 var a = [ 'a', 'b', 'c' ];
-var b = [].concat(a, [ 'foo' ]);
+var b = a.concat([ 'foo' ]);
 ```
 
 ## Installation

packages/babel-plugin-transform-spread/src/index.js

@@ -52,11 +52,11 @@ export default function(api, options) {
         if (!hasSpread(elements)) return;
 
         const nodes = build(elements, scope, state);
-        let first = nodes.shift();
+        const first = nodes.shift();
 
-        if (!t.isArrayExpression(first)) {
-          nodes.unshift(first);
-          first = t.arrayExpression([]);
+        if (nodes.length === 0 && first !== elements[0].argument) {
+          path.replaceWith(first);
+          return;
         }
 
         path.replaceWith(

packages/babel-plugin-transform-spread/test/fixtures/spread/arguments-array/expected.js

@@ -1,5 +1,5 @@
 function foo() {
-  return bar([].concat(Array.prototype.slice.call(arguments)));
+  return bar(Array.prototype.slice.call(arguments));
 }
 
 function bar(one, two, three) {

packages/babel-plugin-transform-spread/test/fixtures/spread/array-literal-first/expected.js

@@ -1 +1 @@
-var lyrics = [].concat(babelHelpers.toConsumableArray(parts), ["head", "and", "toes"]);
+var lyrics = babelHelpers.toConsumableArray(parts).concat(["head", "and", "toes"]);

packages/babel-plugin-transform-spread/test/fixtures/spread/known-rest/expected.js

@@ -3,5 +3,5 @@ function foo() {
     bar[_key] = arguments[_key];
   }
 
-  return [].concat(bar);
+  return bar.concat();
 }

packages/babel-plugin-transform-spread/test/fixtures/spread/single/expected.js

@@ -1 +1 @@
-[].concat(babelHelpers.toConsumableArray(foo));
+babelHelpers.toConsumableArray(foo);