Lazily format LanguageError messages (fix issue 15069).

runtime/lib/mirrors.cc

Index: runtime/lib/mirrors.cc
===================================================================
--- runtime/lib/mirrors.cc	(revision 30304)
+++ runtime/lib/mirrors.cc	(working copy)
@@ -51,7 +51,7 @@
   if (FLAG_use_mirrored_compilation_error && error.IsLanguageError()) {
     // A compilation error that was delayed by lazy compilation.
     const LanguageError& compilation_error = LanguageError::Cast(error);
-    String& message = String::Handle(compilation_error.message());
+    String& message = String::Handle(compilation_error.FormatMessage());
     ThrowMirroredCompilationError(message);
     UNREACHABLE();
   }