Move Maps' back pointers from "transitions" to "constructor" field

test/mjsunit/function-prototype.js

Index: test/mjsunit/function-prototype.js
diff --git a/test/mjsunit/function-prototype.js b/test/mjsunit/function-prototype.js
index 7eac6df1215d675de22554800894b04db570ba90..76ab53cf0741a6bc2cd225ee4979c341c793af50 100644
--- a/test/mjsunit/function-prototype.js
+++ b/test/mjsunit/function-prototype.js
@@ -34,6 +34,9 @@ function TestNonObjectPrototype(value) {
   var f = new F();
   assertEquals(value, F.prototype);
   assertEquals(Object.prototype, f.__proto__);
+  // Test that map transitions don't break anything.
+  F.property = "value";
+  assertEquals(value, F.prototype);
 }
 
 var values = [123, "asdf", true];