Implement rudimentary module linking.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index df3717e4faee7bb650dfb006fcdd32742a14c7e4..c35597c9e0aa86e35861300ecfc692969272f991 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -530,6 +530,8 @@ class Heap {
   MUST_USE_RESULT MaybeObject* AllocateJSObject(
       JSFunction* constructor, PretenureFlag pretenure = NOT_TENURED);
 
+  MUST_USE_RESULT MaybeObject* AllocateJSModule();
+
   // Allocate a JSArray with no elements
   MUST_USE_RESULT MaybeObject* AllocateEmptyJSArray(
       ElementsKind elements_kind,
@@ -821,6 +823,10 @@ class Heap {
   // Allocate a global (but otherwise uninitialized) context.
   MUST_USE_RESULT MaybeObject* AllocateGlobalContext();
 
+  // Allocate a module context.
+  MUST_USE_RESULT MaybeObject* AllocateModuleContext(Context* previous,
+                                                     ScopeInfo* scope_info);
+
   // Allocate a function context.
   MUST_USE_RESULT MaybeObject* AllocateFunctionContext(int length,
                                                        JSFunction* function);