Workaround flow parser bug with spread in arrays (#285)

The current output of

```js
[...a, ...b]
```

is

```js
[...a, , ...b]
```

because flow parses it as

```
ArrayExpression(SpreadExpression, null, SpreadExpression)
```

This is a bug in the flow parser. Until it gets fixed, we can workaround it by deleting the `null` after a `SpreadExpression`.

src/printer.js

@@ -641,6 +641,26 @@ function genericPrintNoParens(path, options, print) {
       if (n.elements.length === 0) {
         parts.push("[]");
       } else {
+        // The flow parser in 0.37 has a bug where
+        //   [ ...a, ...a ]
+        // is being parsed as
+        //   ArrayExpression(SpreadElement, null, SpreadElement)
+        // The issue is that it treats the comma as a null element.
+        //
+        // In order to workaround this bug, we delete the null of a sequence
+        // [SpreadElement, null].
+        //
+        // Note: remove this block when we upgrade to 0.38
+        if (options.useFlowParser) {
+          for (let i = 0; i < n.elements.length; ++i) {
+            const element = n.elements[i];
+            if (element && element.type === 'SpreadElement' &&
+                n.elements[i + 1] === null) {
+              n.elements.splice(i + 1, 1);
+            }
+          }
+        }
+
         // JavaScript allows you to have empty elements in an array which
         // changes its length based on the number of commas. The algorithm
         // is that if the last argument is null, we need to force insert

tests/array_spread/__snapshots__/jsfmt.spec.js.snap

@@ -1,3 +1,21 @@
+exports[`test multiple.js 1`] = `
+"[...a, ...b,];
+[...a, ...b];
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+[ ...a, ...b ];
+[ ...a, ...b ];
+"
+`;
+
+exports[`test multiple.js 2`] = `
+"[...a, ...b,];
+[...a, ...b];
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+[ ...a, ...b ];
+[ ...a, ...b ];
+"
+`;
+
 exports[`test test.js 1`] = `
 "var A = [1,2,3];
 var B = [...A];
@@ -18,3 +36,24 @@ var x: Array<string> = [ \"1\", \"2\" ];
 var y: Array<string> = [ \"3\", ...x ];
 "
 `;
+
+exports[`test test.js 2`] = `
+"var A = [1,2,3];
+var B = [...A];
+var C = [1,2,3];
+B.sort((a, b) => a - b);
+C.sort((a, b) => a - b);
+
+var x: Array<string> = [\'1\', \'2\'];
+var y: Array<string> = [\'3\', ...x];
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+var A = [ 1, 2, 3 ];
+var B = [ ...A ];
+var C = [ 1, 2, 3 ];
+B.sort((a, b) => a - b);
+C.sort((a, b) => a - b);
+
+var x: Array<string> = [ \"1\", \"2\" ];
+var y: Array<string> = [ \"3\", ...x ];
+"
+`;

tests/array_spread/jsfmt.spec.js

@@ -1 +1,2 @@
 run_spec(__dirname);
+run_spec(__dirname, {useFlowParser: false});

tests/array_spread/multiple.js

@@ -0,0 +1,2 @@
+[...a, ...b,];
+[...a, ...b];