Rename references to "external pages" as "snapshot pages" to avoid confusion with the kind of exter…

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index a3edb3011362319997fa5ae110243bdd2083a608..b6945186ce43b5f176b91e1ca3aaefd9028ef927 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -609,9 +609,9 @@ class RawObject {
   friend class RetainingPathVisitor;       // GetClassId
   friend class SkippedCodeFunctions;       // StorePointer
   friend class InstructionsReader;         // tags_ check
-  friend class InstructionsWriter;
-  friend class AssemblyInstructionsWriter;
-  friend class BlobInstructionsWriter;
+  friend class ImageWriter;
+  friend class AssemblyImageWriter;
+  friend class BlobImageWriter;
   friend class SnapshotReader;
   friend class Deserializer;
   friend class SnapshotWriter;
@@ -1214,7 +1214,7 @@ class RawInstructions : public RawObject {
   friend class SkippedCodeFunctions;
   friend class Function;
   friend class InstructionsReader;
-  friend class InstructionsWriter;
+  friend class ImageWriter;
 };