Implement immutable prototype chains

test/mjsunit/regress/regress-crbug-571517.js

Index: test/mjsunit/regress/regress-crbug-571517.js
diff --git a/test/mjsunit/regress/regress-crbug-571517.js b/test/mjsunit/regress/regress-crbug-571517.js
index 03bf76cb5e36924f8d4b15b6c0515fc7715f369e..ca7d7f73ba6a212e56626a65b32ae4416acf84b1 100644
--- a/test/mjsunit/regress/regress-crbug-571517.js
+++ b/test/mjsunit/regress/regress-crbug-571517.js
@@ -11,7 +11,10 @@ function f(a) {
 
 var rec = new Receiver();
 
-var proto = rec.__proto__.__proto__;
+// Formerly, this mutated rec.__proto__.__proto__, but
+// the global object prototype chain is now immutable;
+// not sure if this test now hits the original hazard case.
+var proto = rec.__proto__;
 
 // Initialize prototype chain dependent IC (nonexistent load).
 assertEquals(undefined, f(rec));