[mjsunit] Exit on hitting unreachable code instead of throwing

test/mjsunit/es6/promises.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 16 matching lines @@
 
 // Flags: --allow-natives-syntax
 
 // Make sure we don't rely on functions patchable by monkeys.
 var call = Function.prototype.call.call.bind(Function.prototype.call)
 var getOwnPropertyNames = Object.getOwnPropertyNames;
 var defineProperty = Object.defineProperty;
 var numberPrototype = Number.prototype;
 var symbolIterator = Symbol.iterator;
 
+function assertUnreachable() {
+  %AbortJS("Failure: unreachable");
+}
 
 (function() {
   // Test before clearing global (fails otherwise)
   assertEquals("[object Promise]",
       Object.prototype.toString.call(new Promise(function() {})));
 })();
 
 
 function clear(o) {
   if (o === null || (typeof o !== 'object' && typeof o !== 'function')) return
@@ skipping 957 matching lines @@
   var p = Promise.resolve();
   var callCount = 0;
   defineProperty(p, "constructor", {
     get: function() { ++callCount; return Promise; }
   });
   p.then();
   assertEquals(1, callCount);
 })();
 
 assertAsyncDone()