Fix default class super inheritance (#7732)

Fixes #7683.

packages/babel-plugin-transform-classes/test/fixtures/spec/default-super/exec.js

@@ -0,0 +1,17 @@
+class Test {
+  constructor() {
+    return super.constructor;
+  }
+
+  static test() {
+    return super.constructor;
+  }
+}
+
+// Instances
+expect(Object.getPrototypeOf(Test.prototype)).toBe(Object.prototype);
+expect(new Test()).toBe(Object);
+
+// Static
+expect(Object.getPrototypeOf(Test)).toBe(Function.prototype);
+expect(Test.test()).toBe(Function);