[BUGFIX] Ensure tagForProperty works on class constructors

`tagForProperty` was returning the constant tag for functions, which
meant it would return the constant tag for properties on class
constructors. If a computed property was defined on the class
constructor it would fail because of this.

packages/@ember/-internals/metal/lib/tags.ts

@@ -22,7 +22,7 @@ function makeTag(): TagWrapper<DirtyableTag> {
 }
 
 export function tagForProperty(object: any, propertyKey: string | symbol, _meta?: Meta): Tag {
-  if (typeof object !== 'object' || object === null) {
+  if (typeof object !== 'function' && (typeof object !== 'object' || object === null)) {
     return CONSTANT_TAG;
   }
   let meta = _meta === undefined ? metaFor(object) : _meta;

packages/@ember/-internals/metal/tests/computed_test.js

@@ -82,6 +82,19 @@ moduleFor(
       assert.equal(count, 1, 'should have invoked computed property');
     }
 
+    ['@test computed property can be defined and accessed on a class constructor'](assert) {
+      let Obj = EmberObject.extend();
+      Obj.reopenClass({
+        bar: 123,
+
+        foo: computed(function() {
+          return this.bar;
+        }),
+      });
+
+      assert.equal(Obj.foo, '123', 'should return value');
+    }
+
     ['@test can override volatile computed property'](assert) {
       let obj = {};