Use [[DefineOwnProperty]] to put 'constructor' field on the protoype object.

test/mjsunit/mjsunit.js

Index: test/mjsunit/mjsunit.js
diff --git a/test/mjsunit/mjsunit.js b/test/mjsunit/mjsunit.js
index 558282f52bb39126003503fac3aedeb94f27ecff..fe580f35005a42b4e404c3003092a17118ff3252 100644
--- a/test/mjsunit/mjsunit.js
+++ b/test/mjsunit/mjsunit.js
@@ -104,6 +104,13 @@ function deepEquals(a, b) {
 }
 
 
+function assertSame(expected, found, name_opt) {
+  if (found !== expected) {
+    fail(expected, found, name_opt);
+  }
+}
+
+
 function assertEquals(expected, found, name_opt) {
   if (!deepEquals(found, expected)) {
     fail(expected, found, name_opt);