Use the distinguished class, ApiFailure, to signal when a Dart_Result is

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 478)
+++ runtime/vm/raw_object.h	(working copy)
@@ -37,6 +37,7 @@
   V(Context)                                                                   \
   V(ContextScope)                                                              \
   V(UnhandledException)                                                        \
+  V(ApiFailure)                                                                \
   V(Instance)                                                                  \
     V(Number)                                                                  \
       V(Integer)                                                               \
@@ -588,6 +589,19 @@
 };
 
 
+class RawApiFailure : public RawObject {
+  RAW_HEAP_OBJECT_IMPLEMENTATION(ApiFailure);
+
+  RawObject** from() {
+    return reinterpret_cast<RawObject**>(&ptr()->message_);
+  }
+  RawString* message_;
+  RawObject** to() {
+    return reinterpret_cast<RawObject**>(&ptr()->message_);
+  }
+};
+
+
 class RawInstance : public RawObject {
   RAW_HEAP_OBJECT_IMPLEMENTATION(Instance);
 };