Group lexical context variables for faster look up.

src/contexts.cc

Index: src/contexts.cc
diff --git a/src/contexts.cc b/src/contexts.cc
index 9f39aecd542ca24dbbad3bf7baf1109e853494ba..6bd4fcef11053c6285e02b1a98adda3e4afaf20a 100644
--- a/src/contexts.cc
+++ b/src/contexts.cc
@@ -56,6 +56,19 @@ bool ScriptContextTable::Lookup(Handle<ScriptContextTable> table,
 }
 
 
+bool ScriptContextTable::LookupLexical(Handle<ScriptContextTable> table,
+                                       Handle<String> name) {
+  for (int i = 0; i < table->used(); i++) {
+    Handle<Context> context = GetContext(table, i);
+    DCHECK(context->IsScriptContext());
+    Handle<ScopeInfo> scope_info(ScopeInfo::cast(context->extension()));
+    int slot_index = ScopeInfo::LexicalContextSlotIndex(scope_info, name);
+    if (slot_index >= 0) return true;
+  }
+  return false;
+}
+
+
 Context* Context::declaration_context() {
   Context* current = this;
   while (!current->IsFunctionContext() && !current->IsNativeContext() &&