Improve error messages.

pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
index 28dffb9b4179e487c10bb5602c2ac27a8ad434ed..5e132003d7678dfcc2cc5c541046d2180b065513 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
@@ -749,14 +749,11 @@ class KernelLibraryBuilder
           });
       }
     }
-    if (isExport) {
-      addNit(charOffset,
-          "'$name' is exported from both '${uri}' and '${otherUri}'.");
-    } else {
-      addNit(charOffset,
-          "'$name' is imported from both '${uri}' and '${otherUri}'.");
-    }
-    return new KernelInvalidTypeBuilder(name, charOffset, fileUri);
+    String message = isExport
+        ? "'$name' is exported from both '${uri}' and '${otherUri}'."
+        : "'$name' is imported from both '${uri}' and '${otherUri}'.";
+    addNit(charOffset, message);
+    return new KernelInvalidTypeBuilder(name, charOffset, fileUri, message);
   }
 
   int finishStaticInvocations() {