Make deserialization more regular as prep for deferring canonicalization for types/instances.

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 6667b814116a2d8213adc39cc4f83ff2a4d8a4e0..a23585ef441c9c32f47dac9dbcb0cbb1207e8d8f 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -793,9 +793,12 @@ class RawScript : public RawObject {
 
   RawObject** from() { return reinterpret_cast<RawObject**>(&ptr()->url_); }
   RawString* url_;
-  RawString* source_;
   RawTokenStream* tokens_;
-  RawObject** to() { return reinterpret_cast<RawObject**>(&ptr()->tokens_); }
+  RawObject** to_snapshot() {
+    return reinterpret_cast<RawObject**>(&ptr()->tokens_);
+  }
+  RawString* source_;
+  RawObject** to() { return reinterpret_cast<RawObject**>(&ptr()->source_); }
 
   int32_t line_offset_;
   int32_t col_offset_;