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

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 4770)
+++ runtime/vm/parser.cc	(working copy)
@@ -3310,7 +3310,9 @@
                                Api::NewLocalHandle(url),
                                Api::NewLocalHandle(import_map));
   if (Dart_IsError(result)) {
-    ErrorMsg(token_pos, "library handler failed: %s", Dart_GetError(result));
+    Error& prev_error = Error::Handle();
+    prev_error ^= Api::UnwrapHandle(result);
+    AppendErrorMsg(prev_error, token_pos, "library handler failed");
   }
   return result;
 }
@@ -5254,6 +5256,23 @@
 }
 
 
+RawError* Parser::FormatErrorWithAppend(const Error& prev_error,
+                                        const Script& script,
+                                        intptr_t token_index,
+                                        const char* message_header,
+                                        const char* format,
+                                        va_list args) {
+  const intptr_t kMessageBufferSize = 512;
+  char message_buffer[kMessageBufferSize];
+  FormatMessage(script, token_index, message_header,
+                message_buffer, kMessageBufferSize,
+                format, args);
+  const String& msg1 = String::Handle(String::New(prev_error.ToErrorCString()));

[hausner, 2012/03/01 00:10:16]

Why not use prev_error.message()? There is still a detour through C land in your
version.

[srdjan, 2012/03/01 00:17:34]

There is no Error::message(). Some subclasses implement it, some don't. All
implement ToErrorCString, although some have UNREACHABLE in them.

[turnidge, 2012/03/07 00:41:21]

You should be able to use ToErrorCString on any real Error object.  Nobody is
using UnwindErrors yet, so the UNIMPLEMENTED there doesn't matter.  All other
subclasses of Error have reasonable implementations.

+  const String& msg2 = String::Handle(String::New(message_buffer));
+  return LanguageError::New(String::Handle(String::Concat(msg1, msg2)));
+}
+
+
 RawError* Parser::FormatError(const Script& script,
                               intptr_t token_index,
                               const char* message_header,
@@ -5354,6 +5373,18 @@
 }
 
 
+void Parser::AppendErrorMsg(
+      const Error& prev_error, intptr_t token_index, const char* format, ...) {
+  va_list args;
+  va_start(args, format);
+  const Error& error = Error::Handle(FormatErrorWithAppend(
+      prev_error, script_, token_index, "Error", format, args));
+  va_end(args);
+  Isolate::Current()->long_jump_base()->Jump(1, error);
+  UNREACHABLE();
+}
+
+
 void Parser::Warning(intptr_t token_index, const char* format, ...) {
   if (FLAG_silent_warnings) return;
   va_list args;