Fix Issue 1846: Improve error reporting in presence of imports. With this CL:

runtime/vm/parser.h

 // Copyright (c) 2012, 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_PARSER_H_
 #define VM_PARSER_H_
 
 #include "include/dart_api.h"
 
 #include "vm/ast.h"
@@ skipping 174 matching lines @@
                             va_list args);
 
   // Reports error/warning message at location of current token.
   void ErrorMsg(const char* msg, ...);
   void Warning(const char* msg, ...);
   void Unimplemented(const char* msg);
 
   // Reports error message at given location.
   void ErrorMsg(intptr_t token_index, const char* msg, ...);
   void Warning(intptr_t token_index, const char* msg, ...);
+  // Concatenates two error messages, the previous and the current one.
+  void AppendErrorMsg(
+      const Error& prev_error, intptr_t token_index, const char* format, ...);
+
+  // Same as FormatError, but appends the new error to the 'prev_error'.
+  static RawError* FormatErrorWithAppend(const Error& prev_error,
+                                         const Script& script,
+                                         intptr_t token_index,
+                                         const char* message_header,
+                                         const char* format,
+                                         va_list args);
 
   const Instance& EvaluateConstExpr(AstNode* expr);
   void RunStaticFieldInitializer(const Field& field);
   RawObject* EvaluateConstConstructorCall(
       const Class& type_class,
       const AbstractTypeArguments& type_arguments,
       const Function& constructor,
       ArgumentListNode* arguments);
   AstNode* FoldConstExpr(intptr_t expr_pos, AstNode* expr);
 
@@ skipping 259 matching lines @@
   // code at all points in the try block where an exit from the block is
   // done using 'return', 'break' or 'continue' statements.
   TryBlocks* try_blocks_list_;
 
   DISALLOW_COPY_AND_ASSIGN(Parser);
 };
 
 }  // namespace dart
 
 #endif  // VM_PARSER_H_