Report compile-time error instead of crashing when encountering bad parts.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index 5c2115375e9ad5dc509ab715173f111565e506b0..6c992f6654fd75c32bbefd244316ff96d96e3095 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -8,7 +8,9 @@ import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
 
 import '../combinator.dart' show Combinator;
 
-import '../errors.dart' show internalError;
+import '../errors.dart' show inputError, internalError;
+
+import '../export.dart' show Export;
 
 import '../messages.dart' show warning;
 
@@ -320,11 +322,13 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
 
   void validatePart() {
     if (parts.isNotEmpty) {
-      internalError("Part with parts: $uri");
+      inputError(fileUri, -1,
+          "A file that's a part of a library can't have parts itself.");
     }
     if (exporters.isNotEmpty) {
-      internalError(
-          "${exporters.first.exporter.uri} attempts to export the part $uri.");
+      Export export = exporters.first;
+      inputError(
+          export.fileUri, export.charOffset, "A part can't be exported.");
     }
   }