Introduce the Error object class in the vm. It represents all of the

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
===================================================================
--- runtime/vm/raw_object.cc	(revision 3167)
+++ runtime/vm/raw_object.cc	(working copy)
@@ -392,6 +392,28 @@
 }
 
 
+intptr_t RawError::VisitErrorPointers(RawError* raw_obj,
+                                      ObjectPointerVisitor* visitor) {
+  // Error is an abstract class.
+  UNREACHABLE();
+  return 0;
+}
+
+
+intptr_t RawApiError::VisitApiErrorPointers(
+    RawApiError* raw_obj, ObjectPointerVisitor* visitor) {
+  visitor->VisitPointers(raw_obj->from(), raw_obj->to());
+  return ApiError::InstanceSize();
+}
+
+
+intptr_t RawLanguageError::VisitLanguageErrorPointers(
+    RawLanguageError* raw_obj, ObjectPointerVisitor* visitor) {
+  visitor->VisitPointers(raw_obj->from(), raw_obj->to());
+  return LanguageError::InstanceSize();
+}
+
+
 intptr_t RawUnhandledException::VisitUnhandledExceptionPointers(
     RawUnhandledException* raw_obj, ObjectPointerVisitor* visitor) {
   visitor->VisitPointers(raw_obj->from(), raw_obj->to());
@@ -399,10 +421,10 @@
 }
 
 
-intptr_t RawApiError::VisitApiErrorPointers(
-    RawApiError* raw_obj, ObjectPointerVisitor* visitor) {
+intptr_t RawUnwindError::VisitUnwindErrorPointers(
+    RawUnwindError* raw_obj, ObjectPointerVisitor* visitor) {
   visitor->VisitPointers(raw_obj->from(), raw_obj->to());
-  return ApiError::InstanceSize();
+  return UnwindError::InstanceSize();
 }