Fix stack trace accessor behavior.

test/mjsunit/stack-traces-overflow.js

Index: test/mjsunit/stack-traces-overflow.js
diff --git a/test/mjsunit/stack-traces-overflow.js b/test/mjsunit/stack-traces-overflow.js
index 1c676802101c57d0f0c4d3a165bd90c1746d55bd..fab49d34c8de9844b288530e29020b3d966211a4 100644
--- a/test/mjsunit/stack-traces-overflow.js
+++ b/test/mjsunit/stack-traces-overflow.js
@@ -61,8 +61,8 @@ try {
 function testErrorPrototype(prototype) {
   var object = {};
   object.__proto__ = prototype;
-  object.stack = "123";
-  assertEquals("123", object.stack);
+  object.stack = "123";  // Overwriting stack property fails.
+  assertEquals(prototype.stack, object.stack);
   assertTrue("123" != prototype.stack);
 }
 
@@ -126,6 +126,8 @@ try {
   rec1(0);
 } catch (e) {
   assertEquals(undefined, e.stack);
+  e.stack = "abc";
+  assertEquals("abc", e.stack);
 }
 
 Error.stackTraceLimit = 3;