Last snapshot bits to get working precompiled hello_world on x64.

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index f612073aa625f5b310bc441067282e8e61547096..25aff579a9bd46195c690b84c7252f380f4bb339 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -32,6 +32,7 @@ class Library;
 class Object;
 class PassiveObject;
 class ObjectStore;
+class MegamorphicCache;
 class PageSpace;
 class RawApiError;
 class RawArray;
@@ -339,6 +340,8 @@ class SnapshotReader : public BaseReader {
   ExternalTypedData* DataHandle() { return &data_; }
   TypedData* TypedDataHandle() { return &typed_data_; }
   Code* CodeHandle() { return &code_; }
+  Function* FunctionHandle() { return &function_; }
+  MegamorphicCache* MegamorphicCacheHandle() { return &megamorphic_cache_; }
   Snapshot::Kind kind() const { return kind_; }
   bool snapshot_code() const { return snapshot_code_; }
 
@@ -521,6 +524,8 @@ class SnapshotReader : public BaseReader {
   ExternalTypedData& data_;  // Temporary stream data handle.
   TypedData& typed_data_;  // Temporary typed data handle.
   Code& code_;  // Temporary code handle.
+  Function& function_;  // Temporary function handle.
+  MegamorphicCache& megamorphic_cache_;  // Temporary megamorphic cache handle.
   UnhandledException& error_;  // Error handle.
   intptr_t max_vm_isolate_object_id_;
   ZoneGrowableArray<BackRefNode>* backward_references_;
@@ -780,7 +785,7 @@ class InstructionsWriter : public ZoneAllocated {
                      ReAlloc alloc,
                      intptr_t initial_size)
     : stream_(buffer, alloc, initial_size),
-      next_offset_(0),
+      next_offset_(HeaderSize()),
       instructions_() {
     ASSERT(buffer != NULL);
     ASSERT(alloc != NULL);
@@ -803,6 +808,11 @@ class InstructionsWriter : public ZoneAllocated {
 
   void WriteAssembly();
 
+  intptr_t HeaderSize() {
+    ASSERT(OS::kMaxPreferredCodeAlignment > sizeof(uint64_t));
+    return OS::kMaxPreferredCodeAlignment;
+  }
+
  private:
   struct InstructionsData {
     explicit InstructionsData(RawInstructions* insns)