Revert "Removed default Isolate."

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 2661fa0c1fbf4c36899ae6d0fac0078e2cd5e238..396d68b7fa98ecb4db7c6d09bfc80494cf00d975 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -1659,7 +1659,7 @@ int Shell::Main(int argc, char* argv[]) {
     v8::V8::SetArrayBufferAllocator(&array_buffer_allocator);
   }
   int result = 0;
-  Isolate* isolate = Isolate::New();
+  Isolate* isolate = Isolate::GetCurrent();
 #ifndef V8_SHARED
   v8::ResourceConstraints constraints;
   constraints.ConfigureDefaults(i::OS::TotalPhysicalMemory(),
@@ -1669,7 +1669,6 @@ int Shell::Main(int argc, char* argv[]) {
 #endif
   DumbLineEditor dumb_line_editor(isolate);
   {
-    Isolate::Scope scope(isolate);
     Initialize(isolate);
 #ifdef ENABLE_VTUNE_JIT_INTERFACE
     vTune::InitializeVtuneForV8();