Make fewer assumptions about the layout of api handles.

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 11628)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -113,17 +113,20 @@
 
   // Returns true if the handle holds a Smi.
   static bool IsSmi(Dart_Handle handle) {
+    // TODO(turnidge): Assumes RawObject* is at offset zero.  Fix.

[cshapiro, 2012/08/30 18:18:55]

Is there a straight forward fix?  Since the handle type is ambiguous, this would
require reorganizing handles so they have a similar layout.

[turnidge, 2012/08/30 18:24:30]

I see us always forcing LocalHandle, PersistentHandle, and FPHandle to have a
similar layout prefix.  My only problem here is that LocalHandle hasn't been
defined yet.  To fix this, I would refactor dart_api_state.h and dart_api_impl.h
to make the handle types available at this point.  It's worth hacking up
dart_api_state.h in some way eventually I think.

     RawObject* raw = *(reinterpret_cast<RawObject**>(handle));
     return !raw->IsHeapObject();
   }
 
   // Returns the value of a Smi.
   static intptr_t SmiValue(Dart_Handle handle) {
+    // TODO(turnidge): Assumes RawObject* is at offset zero.  Fix.
     uword value = *(reinterpret_cast<uword*>(handle));
     return Smi::ValueFromRaw(value);
   }
 
   static intptr_t ClassId(Dart_Handle handle) {
+    // TODO(turnidge): Assumes RawObject* is at offset zero.  Fix.
     RawObject* raw = *(reinterpret_cast<RawObject**>(handle));
     if (!raw->IsHeapObject()) {
       return kSmiCid;