Guard against allocation top overflow in ObjectArray_Allocate intrinsic.

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
diff --git a/runtime/vm/intrinsifier_x64.cc b/runtime/vm/intrinsifier_x64.cc
index d1957cef22d4514fe9cd54acd864b4dda2b7eb29..baae3fcf7fe0fef118d1674d5a09a515d674863e 100644
--- a/runtime/vm/intrinsifier_x64.cc
+++ b/runtime/vm/intrinsifier_x64.cc
@@ -57,10 +57,13 @@ bool Intrinsifier::ObjectArray_Allocate(Assembler* assembler) {
   Isolate* isolate = Isolate::Current();
   Heap* heap = isolate->heap();
 
-  // RDI: allocation size.
   __ movq(RAX, Immediate(heap->TopAddress()));
   __ movq(RAX, Address(RAX, 0));
-  __ leaq(RCX, Address(RAX, RDI, TIMES_1, 0));
+
+  // RDI: allocation size.
+  __ movq(RCX, RAX);
+  __ addq(RCX, RDI);
+  __ j(CARRY, &fall_through);
 
   // Check if the allocation fits into the remaining space.
   // RAX: potential new object start.