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 4760)
+++ runtime/vm/parser.cc	(working copy)
@@ -3310,7 +3310,8 @@
                                Api::NewLocalHandle(url),
                                Api::NewLocalHandle(import_map));
   if (Dart_IsError(result)) {
-    ErrorMsg(token_pos, "library handler failed: %s", Dart_GetError(result));
+    AppendErrorMsg(
+        Dart_GetError(result), token_pos, "library handler failed");

[hausner, 2012/02/29 22:56:02]

Dart_GetError() copies the error message out of an error object into a C string,
then you pass it to to a function that copies the C string into a Dart string
which is copied again when the old and new error message concatenated. 

This can probably be done with one less indirection. What if you pass the error
object into AppendErrorMsg()?

[srdjan, 2012/02/29 23:47:04]

Done.

   }
   return result;
 }
@@ -5254,6 +5255,23 @@
 }
 
 
+RawError* Parser::FormatErrorWithAppend(const char* 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));
+  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 +5372,18 @@
 }
 
 
+void Parser::AppendErrorMsg(
+      const char* 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;