make parenthesized array patterns illegal - cc @michaelficarra

test/acorn/tests-babel.js

@@ -2,6 +2,14 @@ var test = require("./driver.js").test;
 var testFail = require("./driver.js").testFail;
 var testAssert = require("./driver.js").testAssert;
 
+testFail("({a}) = 2;", "You're trying to assign to a parenthesized expression, instead of `({ foo }) = {}` use `({ foo } = {})` (1:1)", {
+  ecmaVersion: 6
+});
+
+testFail("([a]) = 2;", "You're trying to assign to a parenthesized expression, instead of `({ foo }) = {}` use `({ foo } = {})` (1:1)", {
+  ecmaVersion: 6
+});
+
 // ES7: Exponentiation Operator
 
 test('a **= 2;', {