turn method literal keys into assignments in loose mode - fixes #1797

test/core/fixtures/transformation/es6.classes-loose/literal-key/actual.js

@@ -0,0 +1,5 @@
+class Foo {
+  "bar"() {
+
+  }
+}

test/core/fixtures/transformation/es6.classes-loose/literal-key/expected.js

@@ -0,0 +1,11 @@
+"use strict";
+
+var Foo = (function () {
+  function Foo() {
+    babelHelpers.classCallCheck(this, Foo);
+  }
+
+  Foo.prototype["bar"] = function bar() {};
+
+  return Foo;
+})();