diff --git a/deps/v8/include/v8-version.h b/deps/v8/include/v8-version.h
index 35984a97ad2e8a..79f057037b4dc7 100644
--- a/deps/v8/include/v8-version.h
+++ b/deps/v8/include/v8-version.h
@@ -11,7 +11,7 @@
 #define V8_MAJOR_VERSION 5
 #define V8_MINOR_VERSION 1
 #define V8_BUILD_NUMBER 281
-#define V8_PATCH_LEVEL 90
+#define V8_PATCH_LEVEL 91
 
 // Use 1 for candidates and 0 otherwise.
 // (Boolean macro values are not supported by all preprocessors.)
diff --git a/deps/v8/src/heap/heap.cc b/deps/v8/src/heap/heap.cc
index 2b5ff9cd1aeaac..eae9695caf09af 100644
--- a/deps/v8/src/heap/heap.cc
+++ b/deps/v8/src/heap/heap.cc
@@ -3166,10 +3166,6 @@ FixedArrayBase* Heap::LeftTrimFixedArray(FixedArrayBase* object,
   DCHECK(!lo_space()->Contains(object));
   DCHECK(object->map() != fixed_cow_array_map());
 
-  // Ensure that the no handle-scope has more than one pointer to the same
-  // backing-store.
-  SLOW_DCHECK(CountHandlesForObject(object) <= 1);
-
   STATIC_ASSERT(FixedArrayBase::kMapOffset == 0);
   STATIC_ASSERT(FixedArrayBase::kLengthOffset == kPointerSize);
   STATIC_ASSERT(FixedArrayBase::kHeaderSize == 2 * kPointerSize);
@@ -4804,6 +4800,49 @@ void Heap::IterateSmiRoots(ObjectVisitor* v) {
   v->Synchronize(VisitorSynchronization::kSmiRootList);
 }
 
+// We cannot avoid stale handles to left-trimmed objects, but can only make
+// sure all handles still needed are updated. Filter out a stale pointer
+// and clear the slot to allow post processing of handles (needed because
+// the sweeper might actually free the underlying page).
+class FixStaleLeftTrimmedHandlesVisitor : public ObjectVisitor {
+ public:
+  explicit FixStaleLeftTrimmedHandlesVisitor(Heap* heap) : heap_(heap) {
+    USE(heap_);
+  }
+
+  void VisitPointer(Object** p) override { FixHandle(p); }
+
+  void VisitPointers(Object** start, Object** end) override {
+    for (Object** p = start; p < end; p++) FixHandle(p);
+  }
+
+ private:
+  inline void FixHandle(Object** p) {
+    HeapObject* current = reinterpret_cast<HeapObject*>(*p);
+    if (!current->IsHeapObject()) return;
+    const MapWord map_word = current->map_word();
+    if (!map_word.IsForwardingAddress() && current->IsFiller()) {
+#ifdef DEBUG
+      // We need to find a FixedArrayBase map after walking the fillers.
+      while (current->IsFiller()) {
+        Address next = reinterpret_cast<Address>(current);
+        if (current->map() == heap_->one_pointer_filler_map()) {
+          next += kPointerSize;
+        } else if (current->map() == heap_->two_pointer_filler_map()) {
+          next += 2 * kPointerSize;
+        } else {
+          next += current->Size();
+        }
+        current = reinterpret_cast<HeapObject*>(next);
+      }
+      DCHECK(current->IsFixedArrayBase());
+#endif  // DEBUG
+      *p = nullptr;
+    }
+  }
+
+  Heap* heap_;
+};
 
 void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   v->VisitPointers(&roots_[0], &roots_[kStrongRootListLength]);
@@ -4824,6 +4863,8 @@ void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   v->Synchronize(VisitorSynchronization::kCompilationCache);
 
   // Iterate over local handles in handle scopes.
+  FixStaleLeftTrimmedHandlesVisitor left_trim_visitor(this);
+  isolate_->handle_scope_implementer()->Iterate(&left_trim_visitor);
   isolate_->handle_scope_implementer()->Iterate(v);
   isolate_->IterateDeferredHandles(v);
   v->Synchronize(VisitorSynchronization::kHandleScope);
@@ -5671,32 +5712,6 @@ void Heap::PrintHandles() {
 
 #endif
 
-#ifdef ENABLE_SLOW_DCHECKS
-
-class CountHandleVisitor : public ObjectVisitor {
- public:
-  explicit CountHandleVisitor(Object* object) : object_(object) {}
-
-  void VisitPointers(Object** start, Object** end) override {
-    for (Object** p = start; p < end; p++) {
-      if (object_ == reinterpret_cast<Object*>(*p)) count_++;
-    }
-  }
-
-  int count() { return count_; }
-
- private:
-  Object* object_;
-  int count_ = 0;
-};
-
-int Heap::CountHandlesForObject(Object* object) {
-  CountHandleVisitor v(object);
-  isolate_->handle_scope_implementer()->Iterate(&v);
-  return v.count();
-}
-#endif
-
 class CheckHandleCountVisitor : public ObjectVisitor {
  public:
   CheckHandleCountVisitor() : handle_count_(0) {}
diff --git a/deps/v8/src/heap/heap.h b/deps/v8/src/heap/heap.h
index bbe1f05e4f2659..2d2029912cd717 100644
--- a/deps/v8/src/heap/heap.h
+++ b/deps/v8/src/heap/heap.h
@@ -1394,9 +1394,6 @@ class Heap {
   void ReportHeapStatistics(const char* title);
   void ReportCodeStatistics(const char* title);
 #endif
-#ifdef ENABLE_SLOW_DCHECKS
-  int CountHandlesForObject(Object* object);
-#endif
 
  private:
   class PretenuringScope;
diff --git a/deps/v8/src/heap/mark-compact.cc b/deps/v8/src/heap/mark-compact.cc
index 9abcd1a4b334a6..16f93f13316bad 100644
--- a/deps/v8/src/heap/mark-compact.cc
+++ b/deps/v8/src/heap/mark-compact.cc
@@ -1374,8 +1374,8 @@ class RootMarkingVisitor : public ObjectVisitor {
   void MarkObjectByPointer(Object** p) {
     if (!(*p)->IsHeapObject()) return;
 
-    // Replace flat cons strings in place.
     HeapObject* object = HeapObject::cast(*p);
+
     MarkBit mark_bit = Marking::MarkBitFrom(object);
     if (Marking::IsBlackOrGrey(mark_bit)) return;
 
diff --git a/deps/v8/src/heap/scavenger.cc b/deps/v8/src/heap/scavenger.cc
index 3f532ead6265f0..dce53554474a57 100644
--- a/deps/v8/src/heap/scavenger.cc
+++ b/deps/v8/src/heap/scavenger.cc
@@ -462,6 +462,7 @@ void ScavengeVisitor::VisitPointers(Object** start, Object** end) {
 void ScavengeVisitor::ScavengePointer(Object** p) {
   Object* object = *p;
   if (!heap_->InNewSpace(object)) return;
+
   Scavenger::ScavengeObject(reinterpret_cast<HeapObject**>(p),
                             reinterpret_cast<HeapObject*>(object));
 }
diff --git a/deps/v8/src/objects-inl.h b/deps/v8/src/objects-inl.h
index b75dd1c969a498..11f4d7498d7558 100644
--- a/deps/v8/src/objects-inl.h
+++ b/deps/v8/src/objects-inl.h
@@ -1261,8 +1261,7 @@ Map* MapWord::ToMap() {
   return reinterpret_cast<Map*>(value_);
 }
 
-
-bool MapWord::IsForwardingAddress() {
+bool MapWord::IsForwardingAddress() const {
   return HAS_SMI_TAG(reinterpret_cast<Object*>(value_));
 }
 
diff --git a/deps/v8/src/objects.h b/deps/v8/src/objects.h
index abced2d4bad50c..d1632c9deb2982 100644
--- a/deps/v8/src/objects.h
+++ b/deps/v8/src/objects.h
@@ -1476,7 +1476,7 @@ class MapWord BASE_EMBEDDED {
   // True if this map word is a forwarding address for a scavenge
   // collection.  Only valid during a scavenge collection (specifically,
   // when all map words are heap object pointers, i.e. not during a full GC).
-  inline bool IsForwardingAddress();
+  inline bool IsForwardingAddress() const;
 
   // Create a map word from a forwarding address.
   static inline MapWord FromForwardingAddress(HeapObject* object);
diff --git a/deps/v8/test/mjsunit/regress/regress-620553.js b/deps/v8/test/mjsunit/regress/regress-620553.js
new file mode 100644
index 00000000000000..461b9bb189e559
--- /dev/null
+++ b/deps/v8/test/mjsunit/regress/regress-620553.js
@@ -0,0 +1,17 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-gc
+
+var o0 = [];
+var o1 = [];
+var cnt = 0;
+o1.__defineGetter__(0, function() {
+  if (cnt++ > 2) return;
+  o0.shift();
+  gc();
+  o0.push(0);
+  o0.concat(o1);
+});
+o1[0];
diff --git a/deps/v8/test/mjsunit/regress/regress-621869.js b/deps/v8/test/mjsunit/regress/regress-621869.js
new file mode 100644
index 00000000000000..db34064457cc30
--- /dev/null
+++ b/deps/v8/test/mjsunit/regress/regress-621869.js
@@ -0,0 +1,18 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-gc
+
+var o0 = [];
+var o1 = [];
+var cnt = 0;
+var only_scavenge = true;
+o1.__defineGetter__(0, function() {
+  if (cnt++ > 2) return;
+  o0.shift();
+  gc(only_scavenge);
+  o0.push((64));
+  o0.concat(o1);
+});
+o1[0];