Cross-link descriptor when building from byte strings

This performs more testing for field descriptors built from byte
strings too, but that's mostly incidental. The chief intent is to
check that cross-linking occurs.

csharp/src/Google.Protobuf.Test/Reflection/DescriptorsTest.cs

@@ -71,7 +71,6 @@ namespace Google.Protobuf.Reflection
 
         private void TestFileDescriptor(FileDescriptor file, FileDescriptor importedFile, FileDescriptor importedPublicFile)
         {
-
             Assert.AreEqual("unittest_proto3.proto", file.Name);
             Assert.AreEqual("protobuf_unittest3", file.Package);
 
@@ -214,29 +213,61 @@ namespace Google.Protobuf.Reflection
         }
 
         [Test]
-        public void FieldDescriptor()
+        public void FieldDescriptor_GeneratedCode()
         {
-            MessageDescriptor messageType = TestAllTypes.Descriptor;
-            FieldDescriptor primitiveField = messageType.FindDescriptor<FieldDescriptor>("single_int32");
-            FieldDescriptor enumField = messageType.FindDescriptor<FieldDescriptor>("single_nested_enum");
-            FieldDescriptor messageField = messageType.FindDescriptor<FieldDescriptor>("single_foreign_message");
+            TestFieldDescriptor(UnittestProto3Reflection.Descriptor, TestAllTypes.Descriptor, ForeignMessage.Descriptor, ImportMessage.Descriptor);
+        }
+
+        [Test]
+        public void FieldDescriptor_BuildFromByteStrings()
+        {
+            // The descriptors have to be supplied in an order such that all the
+            // dependencies come before the descriptors depending on them.
+            var descriptorData = new List<ByteString>
+            {
+                UnittestImportPublicProto3Reflection.Descriptor.Proto.ToByteString(),
+                UnittestImportProto3Reflection.Descriptor.Proto.ToByteString(),
+                UnittestProto3Reflection.Descriptor.Proto.ToByteString()
+            };
+            var converted = FileDescriptor.BuildFromByteStrings(descriptorData);
+            TestFieldDescriptor(
+                converted[2],
+                converted[2].FindTypeByName<MessageDescriptor>("TestAllTypes"),
+                converted[2].FindTypeByName<MessageDescriptor>("ForeignMessage"),
+                converted[1].FindTypeByName<MessageDescriptor>("ImportMessage"));
+        }
+
+        public void TestFieldDescriptor(
+            FileDescriptor unitTestProto3Descriptor,
+            MessageDescriptor testAllTypesDescriptor,
+            MessageDescriptor foreignMessageDescriptor,
+            MessageDescriptor importMessageDescriptor)
+        {
+            FieldDescriptor primitiveField = testAllTypesDescriptor.FindDescriptor<FieldDescriptor>("single_int32");
+            FieldDescriptor enumField = testAllTypesDescriptor.FindDescriptor<FieldDescriptor>("single_nested_enum");
+            FieldDescriptor foreignMessageField = testAllTypesDescriptor.FindDescriptor<FieldDescriptor>("single_foreign_message");
+            FieldDescriptor importMessageField = testAllTypesDescriptor.FindDescriptor<FieldDescriptor>("single_import_message");
 
             Assert.AreEqual("single_int32", primitiveField.Name);
             Assert.AreEqual("protobuf_unittest3.TestAllTypes.single_int32",
                             primitiveField.FullName);
             Assert.AreEqual(1, primitiveField.FieldNumber);
-            Assert.AreEqual(messageType, primitiveField.ContainingType);
-            Assert.AreEqual(UnittestProto3Reflection.Descriptor, primitiveField.File);
+            Assert.AreEqual(testAllTypesDescriptor, primitiveField.ContainingType);
+            Assert.AreEqual(unitTestProto3Descriptor, primitiveField.File);
             Assert.AreEqual(FieldType.Int32, primitiveField.FieldType);
             Assert.IsNull(primitiveField.Proto.Options);
             
             Assert.AreEqual("single_nested_enum", enumField.Name);
             Assert.AreEqual(FieldType.Enum, enumField.FieldType);
-            Assert.AreEqual(messageType.EnumTypes[0], enumField.EnumType);
+            Assert.AreEqual(testAllTypesDescriptor.EnumTypes[0], enumField.EnumType);
+
+            Assert.AreEqual("single_foreign_message", foreignMessageField.Name);
+            Assert.AreEqual(FieldType.Message, foreignMessageField.FieldType);
+            Assert.AreEqual(foreignMessageDescriptor, foreignMessageField.MessageType);
 
-            Assert.AreEqual("single_foreign_message", messageField.Name);
-            Assert.AreEqual(FieldType.Message, messageField.FieldType);
-            Assert.AreEqual(ForeignMessage.Descriptor, messageField.MessageType);
+            Assert.AreEqual("single_import_message", importMessageField.Name);
+            Assert.AreEqual(FieldType.Message, importMessageField.FieldType);
+            Assert.AreEqual(importMessageDescriptor, importMessageField.MessageType);
         }
 
         [Test]

csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs

@@ -344,6 +344,7 @@ namespace Google.Protobuf.Reflection
                 FileDescriptor descriptor = new FileDescriptor(
                     data, proto, dependencies, pool,
                     allowUnknownDependencies: false, generatedCodeInfo: null);
+                descriptor.CrossLink();
                 descriptors.Add(descriptor);
                 if (descriptorsByName.ContainsKey(descriptor.Name))
                 {