Codify the assumption that %GetArrayKeys can return only a single interval starting at zero

src/runtime.cc

 // Copyright 2012 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 9551 matching lines @@
   Handle<FixedArray> single_interval = isolate->factory()->NewFixedArray(2);
   // -1 means start of array.
   single_interval->set(0, Smi::FromInt(-1));
   Handle<Object> number = isolate->factory()->NewNumberFromUint(length);
   single_interval->set(1, *number);
   return isolate->factory()->NewJSArrayWithElements(single_interval);
 }
 
 
 // Returns an array that tells you where in the [0, length) interval an array
-// might have elements.  Can either return keys (positive integers) or
-// intervals (pair of a negative integer (-start-1) followed by a
-// positive (length)) or undefined values.
+// might have elements.  Can either return keys (positive integers or undefined)
+// or a single interval (pair of -1, positive length).
 // Intervals can span over some keys that are not in the object.
 RUNTIME_FUNCTION(MaybeObject*, Runtime_GetArrayKeys) {
   HandleScope scope(isolate);
   ASSERT(args.length() == 2);
   CONVERT_ARG_HANDLE_CHECKED(JSObject, array, 0);
   CONVERT_NUMBER_CHECKED(uint32_t, length, Uint32, args[1]);
   if (array->elements()->IsDictionary()) {
     Handle<FixedArray> keys = isolate->factory()->empty_fixed_array();
     for (Handle<Object> p = array;
          !p->IsNull();
@@ skipping 3417 matching lines @@
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(Heap::kNoGCFlags,
                                        "Runtime::PerformGC");
   }
 }
 
 
 } }  // namespace v8::internal