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

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.source_library_builder;
 
 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;
 
 import '../import.dart' show Import;
 
 import 'source_loader.dart' show SourceLoader;
 
 import '../builder/scope.dart' show Scope;
 
 import '../builder/builder.dart'
@@ skipping 291 matching lines @@
     }
     return null;
   }
 
   void addImplementationBuilder(String name, Builder builder, int charOffset) {
     implementationBuilders.add([name, builder, charOffset]);
   }
 
   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.");
     }
   }
 
   void includeParts() {
     for (SourceLibraryBuilder<T, R> part in parts.toList()) {
       includePart(part);
     }
   }
 
   void includePart(SourceLibraryBuilder<T, R> part) {
@@ skipping 172 matching lines @@
   }
 
   /// Called to register [procedure] as a factory whose types are collected in
   /// [factoryDeclaration]. Later, once the class has been built, we can
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 }