Introduce per-isolate assert scopes and API to guard JS execution.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 0418506773314fb4fc81116963bd52e50f0edd5e..16eebf06d89434dfdeaae5cfc5801453f3e4aeda 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -6514,6 +6514,32 @@ void Isolate::Exit() {
 }
 
 
+Isolate::DisallowJavascriptExecutionScope::DisallowJavascriptExecutionScope(
+    Isolate* isolate) {
+  i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);
+  internal_ = reinterpret_cast<void*>(
+      new i::DisallowJavascriptExecution(i_isolate));
+}
+
+
+Isolate::DisallowJavascriptExecutionScope::~DisallowJavascriptExecutionScope() {
+  delete reinterpret_cast<i::DisallowJavascriptExecution*>(internal_);
+}
+
+
+Isolate::AllowJavascriptExecutionScope::AllowJavascriptExecutionScope(
+    Isolate* isolate) {
+  i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);
+  internal_ = reinterpret_cast<void*>(
+      new i::AllowJavascriptExecution(i_isolate));
+}
+
+
+Isolate::AllowJavascriptExecutionScope::~AllowJavascriptExecutionScope() {
+  delete reinterpret_cast<i::AllowJavascriptExecution*>(internal_);
+}
+
+
 void Isolate::GetHeapStatistics(HeapStatistics* heap_statistics) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
   if (!isolate->IsInitialized()) {