Forbid error with mutated key

Because we rewrite the order to have the key before the other
attributes, it’s possible to have the incorrect value for `key` if the
other attribute mutates the key value.  i.e.

```js
var i = 0;
function render() {
return <div attr={i++} key={i++} />;
}
```

Should have `key=1`, but it would actually have `key=0`

src/helpers/extract-open-arguments.js

@@ -61,6 +61,10 @@ export default function extractOpenArguments(path, plugin) {
 
       // If it's not a literal key, we must assign it in the statics array.
       if (!literal) {
+        if (attrs.length) {
+          throw attribute.buildCodeFrameError("Key should always be the first computed attribute.");
+        }
+
         if (!value.isIdentifier()) {
           node = value.scope.maybeGenerateMemoised(node);
           key = t.assignmentExpression("=", node, key);

test/fixtures/error/key/other-attr-updates-key/actual.js

@@ -0,0 +1,3 @@
+function render() {
+  return <div attr={i++} key={i++} />;
+}

test/fixtures/error/key/other-attr-updates-key/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Key should always be the first computed attribute."
+}

test/fixtures/error/key/spread-attributes-mutated-key/actual.js

@@ -0,0 +1,3 @@
+function render() {
+  return <div {...props} key={props.key++} />
+}

test/fixtures/error/key/spread-attributes-mutated-key/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Key should always be the first computed attribute."
+}