Improve semantics of 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 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'
     show
         Builder,
         ClassBuilder,
@@ skipping 305 matching lines @@
           "A file that's a part of a library can't have parts itself.");
     }
     if (exporters.isNotEmpty) {
       Export export = exporters.first;
       inputError(
           export.fileUri, export.charOffset, "A part can't be exported.");
     }
   }
 
   void includeParts() {
+    Set<Uri> seenParts = new Set<Uri>();
     for (SourceLibraryBuilder<T, R> part in parts.toList()) {
       if (part == this) {
         addCompileTimeError(-1, "A file can't be a part of itself.");
+      } else if (seenParts.add(part.fileUri)) {
+        includePart(part);
       } else {
-        includePart(part);
+        addCompileTimeError(
+            -1, "Can't use '${part.fileUri}' as a part more than once.");
       }
     }
   }
 
   void includePart(SourceLibraryBuilder<T, R> part) {
     if (name != null) {
       if (!part.isPart) {
-        warning(
-            part.fileUri,
+        addCompileTimeError(
             -1,
-            "Has no 'part of' declaration but is used as "
-            "a part by ${name} ($uri).");
+            "Can't use ${part.fileUri} as a part, because it has no 'part of'"
+            " declaration.");
         parts.remove(part);
         return;
       }
       if (part.partOfName != name && part.partOfUri != uri) {
         String partName = part.partOfName ?? "${part.partOfUri}";
         String myName = name == null ? "'$uri'" : "'${name}' ($uri)";
-        warning(part.fileUri, -1,
-            "Is part of '$partName' but is used as a part by $myName.");
-        parts.remove(part);
-        return;
+        addWarning(
+            -1,
+            "Using '${part.fileUri}' as part of '$myName' but it's 'part of'"
+            " declaration says '$partName'.");
+        // The part is still included.
       }
     }
     part.members.forEach((String name, Builder builder) {
       if (builder.next != null) {
         assert(builder.next.next == null);
         addBuilder(name, builder.next, builder.next.charOffset);
       }
       addBuilder(name, builder, builder.charOffset);
     });
     types.addAll(part.types);
@@ skipping 157 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;
   }
 }