Explain requirements on fields accessed from generated code.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index ee64a06b6e955facb5e79a0d9f500867dd5614d0..9bffb215b806dac03928c360b5f1712b9532d839 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -254,8 +254,12 @@ class Scavenger {
 
   intptr_t NewSizeInWords(intptr_t old_size_in_words) const;
 
-  // Current allocation top and end. These values are being accessed directly
-  // from generated code.
+  // Accessed from generated code.
+  // ** This block of fields must come first! **
+  // For AOT cross-compilation, we rely on these members having the same offsets
+  // in SIMARM(IA32) and ARM, and the same offsets in SIMARM64(X64) and ARM64.
+  // We use only word-sized fields to avoid differences in struct packing on the
+  // different architectures. See also CheckOffsets in dart.cc.
   uword top_;
   uword end_;