Make fewer assumptions about the layout of api handles.

runtime/vm/dart_api_impl.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_DART_API_IMPL_H_
 #define VM_DART_API_IMPL_H_
 
 #include "vm/allocation.h"
 #include "vm/object.h"
 
@@ skipping 95 matching lines @@
       Dart_Handle object);
 
   // Casts the internal Isolate* type to the external Dart_Isolate type.
   static Dart_Isolate CastIsolate(Isolate* isolate);
 
   // Gets the handle used to designate successful return.
   static Dart_Handle Success(Isolate* isolate);
 
   // Returns true if the handle holds a Smi.
   static bool IsSmi(Dart_Handle handle) {
+    // TODO(turnidge): Assumes RawObject* is at offset zero.  Fix.
     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;
     }
     return raw->GetClassId();
   }
 
   // Generates a handle used to designate an error return.
   static Dart_Handle NewError(const char* format, ...);
 
@@ skipping 33 matching lines @@
   }
  private:
   Isolate* saved_isolate_;
 
   DISALLOW_COPY_AND_ASSIGN(IsolateSaver);
 };
 
 }  // namespace dart.
 
 #endif  // VM_DART_API_IMPL_H_