Make the "sticky error" an Error instead of a String.

runtime/vm/parser.h

Index: runtime/vm/parser.h
===================================================================
--- runtime/vm/parser.h	(revision 3366)
+++ runtime/vm/parser.h	(working copy)
@@ -80,15 +80,13 @@
 
   static void ParseFunction(ParsedFunction* parsed_function);
 
-  // Format an error or warning message into the message_buffer.
+  // Build an error object containing a formatted error or warning message.
   // A null script means no source and a negative token_index means no position.
-  static void FormatMessage(const Script& script,
-                            intptr_t token_index,
-                            const char* message_header,
-                            char* message_buffer,
-                            intptr_t message_buffer_size,
-                            const char* format,
-                            va_list args);
+  static RawError* FormatError(const Script& script,
+                               intptr_t token_index,
+                               const char* message_header,
+                               const char* format,
+                               va_list args);
 
  private:
   struct Block;
@@ -172,6 +170,16 @@
     Function& constructor,
     const AbstractTypeArguments& type_arguments);
 
+  // Format an error or warning message into the message_buffer.
+  // A null script means no source and a negative token_index means no position.
+  static void FormatMessage(const Script& script,
+                            intptr_t token_index,
+                            const char* message_header,
+                            char* message_buffer,
+                            intptr_t message_buffer_size,
+                            const char* format,
+                            va_list args);
+
   // Reports error message at location of current token.
   void ErrorMsg(const char* msg, ...);
   void ErrorMsg(intptr_t token_index, const char* msg, ...);