Make AstGraphBuilder::function_context a SetOncePointer again.

src/compiler/ast-graph-builder.cc

Index: src/compiler/ast-graph-builder.cc
diff --git a/src/compiler/ast-graph-builder.cc b/src/compiler/ast-graph-builder.cc
index 30bdcfb29702987ee000983ae7aa011ae28c19d1..d0043e11bc6bdbe46acef715e8adaa84be0f45a1 100644
--- a/src/compiler/ast-graph-builder.cc
+++ b/src/compiler/ast-graph-builder.cc
@@ -379,7 +379,6 @@ AstGraphBuilder::AstGraphBuilder(Zone* local_zone, CompilationInfo* info,
       globals_(0, local_zone),
       execution_control_(nullptr),
       execution_context_(nullptr),
-      function_context_(nullptr),
       input_buffer_size_(0),
       input_buffer_(nullptr),
       exit_control_(nullptr),
@@ -400,8 +399,10 @@ Node* AstGraphBuilder::GetFunctionClosure() {
 
 
 Node* AstGraphBuilder::GetFunctionContext() {
-  DCHECK(function_context_ != nullptr);
-  return function_context_;
+  if (!function_context_.is_set()) {
+    function_context_.set(NewOuterContextParam());
+  }
+  return function_context_.get();
 }
 
 
@@ -442,8 +443,8 @@ bool AstGraphBuilder::CreateGraph() {
   }
 
   // Initialize the incoming context.
-  function_context_ = NewOuterContextParam();
-  ContextScope incoming(this, scope, function_context_);
+  Node* outer_context = GetFunctionContext();
+  ContextScope incoming(this, scope, outer_context);
 
   // Build receiver check for sloppy mode if necessary.
   // TODO(mstarzinger/verwaest): Should this be moved back into the CallIC?
@@ -456,7 +457,7 @@ bool AstGraphBuilder::CreateGraph() {
   if (heap_slots > 0) {
     // Push a new inner context scope for the function.
     Node* closure = GetFunctionClosure();
-    Node* inner_context = BuildLocalFunctionContext(function_context_, closure);
+    Node* inner_context = BuildLocalFunctionContext(outer_context, closure);
     ContextScope top_context(this, scope, inner_context);
     CreateGraphBody();
   } else {