Testcase for user internal props

Also tag Date _value access test case as custom.

ecmascript-testcases/test-dev-internal-key-access.js

@@ -6,6 +6,12 @@
  *  buffer values nor the ability to create buffer values.
  */
 
+/*---
+{
+    "custom": true
+}
+---*/
+
 /*===
 date: 1970-01-01T00:02:03.456Z
 using Duktape.Buffer, date \xFFvalue: 123456

ecmascript-testcases/test-dev-internal-property-basics.js

@@ -0,0 +1,65 @@
+/*
+ *  Since Duktape 0.12.0, user code can also employ internal properties.
+ *  This testcase exercises a few basics of that.
+ */
+
+/*---
+{
+    "custom": true
+}
+---*/
+
+/*===
+for-in: foo
+JSON.stringify: {"foo":1}
+JX encoded: {foo:1}
+Object.keys: foo
+Object.getOwnPropertyNames: foo,bar
+direct access: 3
+===*/
+
+function test() {
+    var internalKey;
+    var obj;
+
+    // 'foo' is enumerable, writable, configurable
+    var obj = { foo: 1 };
+
+    // 'bar' is not enumerable
+    Object.defineProperty(obj, 'bar', {
+        value: 2, writable: true, enumerable: false, configurable: true
+    });
+
+    // Internal key \xFF\xFFabc is in principle enumerable, but because
+    // internal keys have special behavior, it is never enumerated.
+    internalKey = Duktape.dec('hex', 'ffff616263');  // \xFF\xFFabc
+    obj[internalKey] = 3;
+
+    // For-in only lists enumerable keys
+    for (k in obj) {
+        print('for-in:', k);
+    }
+
+    // JSON.stringify() also only lists enumerable keys
+    print('JSON.stringify:', JSON.stringify(obj));
+
+    // Even the Duktape-specific JX format doesn't list internal keys
+    print('JX encoded:', Duktape.enc('jx', obj));
+
+    // Object.keys() only returns enumerable keys (but not internal keys,
+    // even if they're technically enumerable)
+    print('Object.keys:', Object.keys(obj));
+
+    // Object.getOwnPropertyNames() returns also non-enumerable keys,
+    // but not internal keys
+    print('Object.getOwnPropertyNames:', Object.getOwnPropertyNames(obj));
+
+    // Direct property access to the internal property still works.
+    print('direct access:', obj[internalKey]);
+}
+
+try {
+    test();
+} catch (e) {
+    print(e);
+}