Add d8 functionality for switching between realms (a.k.a. contexts)

test/mjsunit/bugs/bug-proto.js

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-// Flags: --allow-natives-syntax --parallel-recompilation
+var realmA = Realm.current();
+var realmB = Realm.create();
+assertEquals(0, realmA);
+assertEquals(1, realmB);
 
-function f(foo) { return foo.bar(); }
+// The global objects match the realms' this binding.
+assertSame(this, Realm.global(realmA));
+assertSame(Realm.eval(realmB, "this"), Realm.global(realmB));
+assertFalse(this === Realm.global(realmB));
 
-var o = {};
-o.__proto__ = { __proto__: { bar: function() { return 1; } } };
+// The global object is not accessible cross-realm.
+var x = 3;
+Realm.shared = this;
+assertThrows("Realm.eval(realmB, 'x')");
+assertSame(undefined, Realm.eval(realmB, "this.x"));
+assertSame(undefined, Realm.eval(realmB, "Realm.shared.x"));
 
-assertEquals(1, f(o));
-assertEquals(1, f(o));
+Realm.eval(realmB, "Realm.global(0).y = 1");
+assertThrows("y");
+assertSame(undefined, this.y);
 
-%OptimizeFunctionOnNextCall(f, "parallel");
-assertEquals(1, f(o));
-// Change the prototype chain during optimization.
-o.__proto__.__proto__ = { bar: function() { return 2; } };
+// Can get or set other objects' properties cross-realm.
+var p = {a: 1};
+var o = {__proto__: p, b: 2};
+Realm.shared = o;
+assertSame(1, Realm.eval(realmB, "Realm.shared.a"));
+assertSame(2, Realm.eval(realmB, "Realm.shared.b"));
 
-%WaitUntilOptimized(f);
+// Cannot get or set a prototype cross-realm.
+assertSame(undefined, Realm.eval(realmB, "Realm.shared.__proto__"));
 
-assertEquals(2, f(o));
+Realm.eval(realmB, "Realm.shared.__proto__ = {c: 3}");
+assertSame(1, o.a);
+assertSame(undefined, o.c);