Use %DebugGetProperty in debug mirror to check for Promise.

test/mjsunit/es6/mirror-promises.js

Index: test/mjsunit/es6/mirror-promises.js
diff --git a/test/mjsunit/es6/mirror-promises.js b/test/mjsunit/es6/mirror-promises.js
index a3e4a0d18e4152ccaefbd319f3a19e6c087687b4..bce26f4f92f6393027c84cbcb50e0ff9328becaf 100644
--- a/test/mjsunit/es6/mirror-promises.js
+++ b/test/mjsunit/es6/mirror-promises.js
@@ -39,7 +39,8 @@ function testPromiseMirror(promise, status, value) {
   assertEquals("Object", mirror.className());
   assertEquals("#<Promise>", mirror.toText());
   assertSame(promise, mirror.value());
-  assertEquals(value, mirror.promiseValue());
+  assertTrue(mirror.promiseValue() instanceof debug.Mirror);
+  assertEquals(value, mirror.promiseValue().value());
 
   // Parse JSON representation and check.
   var fromJSON = eval('(' + json + ')');
@@ -48,7 +49,7 @@ function testPromiseMirror(promise, status, value) {
   assertEquals('function', refs.lookup(fromJSON.constructorFunction.ref).type);
   assertEquals('Promise', refs.lookup(fromJSON.constructorFunction.ref).name);
   assertEquals(status, fromJSON.status);
-  assertEquals(value, fromJSON.promiseValue);
+  assertEquals(value, refs.lookup(fromJSON.promiseValue.ref).value);
 }
 
 // Test a number of different promises.
@@ -86,4 +87,4 @@ assertEquals(2, ip[1].value().value());
 var m3 = debug.MakeMirror(new Promise(function(resolve, reject) { }));
 ip = m3.internalProperties();
 assertEquals("pending", ip[0].value().value());
-assertEquals("undefined", typeof(ip[1].value().value()));
+assertEquals("undefined", typeof(ip[1].value().value()));