Standardize error printing a bit by moving it to the Error object and

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 3218)
+++ runtime/vm/object.h	(working copy)
@@ -2189,6 +2189,9 @@
 
 
 class Error : public Object {
+ public:
+  virtual const char* ToErrorCString() const;
+
  private:
   HEAP_OBJECT_IMPLEMENTATION(Error, Object);
 };
@@ -2208,6 +2211,8 @@
   static RawApiError* New(const String& message,
                           Heap::Space space = Heap::kNew);
 
+  virtual const char* ToErrorCString() const;
+
  private:
   void set_message(const String& message) const;
 
@@ -2230,6 +2235,8 @@
   static RawLanguageError* New(const String& message,
                                Heap::Space space = Heap::kNew);
 
+  virtual const char* ToErrorCString() const;
+
  private:
   void set_message(const String& message) const;
 
@@ -2258,6 +2265,8 @@
                                     const Instance& stacktrace,
                                     Heap::Space space = Heap::kNew);
 
+  virtual const char* ToErrorCString() const;
+
  private:
   void set_exception(const Instance& exception) const;
   void set_stacktrace(const Instance& stacktrace) const;
@@ -2281,6 +2290,8 @@
   static RawUnwindError* New(const String& message,
                              Heap::Space space = Heap::kNew);
 
+  virtual const char* ToErrorCString() const;
+
  private:
   void set_message(const String& message) const;