Fix some rest optimization errors.

Commit message edited by @jmm.

packages/babel-plugin-transform-es2015-parameters/src/rest.js

@@ -60,6 +60,12 @@ let memberExpressionOptimisationVisitor = {
       let {parentPath} = path;
       let grandparentPath = parentPath.parentPath;
 
+      // ex: [rest[0]] = [rest[1]]
+      if (grandparentPath.isLVal()) {
+        state.deopted = true;
+        return;
+      }
+
       // ex: args[0]
       if (
         parentPath.isMemberExpression({ computed: true, object: node }) &&
@@ -68,7 +74,8 @@ let memberExpressionOptimisationVisitor = {
         !(
           grandparentPath.isAssignmentExpression() &&
           parentPath.node === grandparentPath.node.left
-        )
+        ) &&
+        !grandparentPath.isForInStatement()
       ) {
         // if we know that this member expression is referencing a number then
         // we can safely optimise it

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-member-expression-deoptimisation/expected.js

@@ -83,10 +83,12 @@ function swap() {
     rest[_key9] = arguments[_key9];
   }
 
-  [rest[0], rest[1]] = [rest[1], rest[0]];
+  var _ref = [rest[1], rest[0]];
+  rest[0] = _ref[0];
+  rest[1] = _ref[1];
 }
 
-function x() {
+function forIn() {
   for (var _len10 = arguments.length, rest = Array(_len10), _key10 = 0; _key10 < _len10; _key10++) {
     rest[_key10] = arguments[_key10];
   }