use a different helper if a class contains class methods to avoid non-enumerability and delegation to es6.properties.computed transformer - fixes #984, closes #986

test/fixtures/transformation/es6-classes/computed-methods/exec.js

@@ -0,0 +1,17 @@
+const sym = Symbol();
+
+class Foo {
+  [sym] () {
+    return 1;
+  }
+}
+
+class Bar extends Foo {
+  [sym] () {
+    return super[sym]() + 2;
+  }
+}
+
+let i = new Bar();
+
+assert.equal(i[sym](), 3);