Fix proxified service classes not lazy-initializing event interface

This issue was introduced by e0bb314.
The root cause was that the Object.assign method used in that commit did
not actually copy the given object as expected. The fix is to modify the
original, as it is a safe copy already.

Change-Id: Iabf5a439358f4bcdf3551937d43b3646c2c8b29d

src/tabris/util.js

@@ -135,10 +135,11 @@ export function proxify(getTarget) {
   const fakeTarget = {};
   Object.keys(traps).forEach(trap => handler[trap] = (_, ...args) => {
     const result = Reflect[trap](getTarget(), ...args);
-    if (trap === 'getOwnPropertyDescriptor') {
+    if (trap === 'getOwnPropertyDescriptor' && result) {
       // The VM throws if a property is configurable or non-existing
       // on fakeTarget, but not reported as such here
-      return Object.assign({}, result, {configurable: true});
+      result.configurable = true;
+      return result;
     }
     return traps[trap] ? true : result;
   });

test/tabris/util.test.js

@@ -1,4 +1,4 @@
-import {expect} from '../test';
+import {expect, spy} from '../test';
 import {
   pick,
   omit,
@@ -12,6 +12,7 @@ import {
 import Composite from '../../src/tabris/widgets/Composite';
 import {mockTabris} from '../tabris-mock';
 import ClientMock from './ClientMock';
+import NativeObject from '../../src/tabris/NativeObject';
 
 describe('util', function() {
 
@@ -229,13 +230,29 @@ describe('util', function() {
       expect(proxy instanceof Test).to.be.false;
     });
 
-    it('Does not crash Object.keys', function() {
+    it('does not crash Object.keys', function() {
       mockTabris(new ClientMock());
       const original = new Composite({left: 23});
       proxy = proxify(() => original);
       expect(() => Object.keys(proxy)).not.to.throw(Error);
     });
 
+    it('does support lazy initialized event API', function() {
+      mockTabris(new ClientMock());
+      const TestClass = class extends NativeObject {
+        get _nativeType() { return 'bar'; }
+      };
+      NativeObject.defineEvent(TestClass.prototype, 'foo', {});
+      const instance = new TestClass();
+      const testProxy = proxify(() => instance);
+      const listener = spy();
+
+      testProxy.onFoo(listener);
+      testProxy.onFoo.trigger();
+
+      expect(listener).to.have.been.calledOnce;
+    });
+
   });
 
 });