Change where we rename Descriptor.cs to DescriptorProtoFile.cs.

We now do this in protoc instead of the generation simpler.

Benefits:
- Generation script is simpler
- Detection is simpler as we now only need to care about one filename
- The embedded descriptor knows itself as "google/protobuf/descriptor.proto" avoiding dependency issues

This PR also makes the "invalid dependency" exception clearer in terms of expected and actual dependencies.
diff --git a/csharp/generate_protos.sh b/csharp/generate_protos.sh
index 962f2a9..da6f940 100755
--- a/csharp/generate_protos.sh
+++ b/csharp/generate_protos.sh
@@ -37,10 +37,8 @@
 
 # Descriptor proto
 # TODO(jonskeet): Remove fixup
-cp src/google/protobuf/descriptor.proto src/google/protobuf/descriptor_proto_file.proto
 $PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf/Reflection \
-    src/google/protobuf/descriptor_proto_file.proto
-rm src/google/protobuf/descriptor_proto_file.proto
+    src/google/protobuf/descriptor.proto
 
 $PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf/WellKnownTypes \
     src/google/protobuf/any.proto \
diff --git a/csharp/src/Google.Protobuf/Reflection/DescriptorProtoFile.cs b/csharp/src/Google.Protobuf/Reflection/DescriptorProtoFile.cs
index 39cb38f..f9158ca 100644
--- a/csharp/src/Google.Protobuf/Reflection/DescriptorProtoFile.cs
+++ b/csharp/src/Google.Protobuf/Reflection/DescriptorProtoFile.cs
@@ -1,5 +1,5 @@
 // Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/descriptor_proto_file.proto
+// source: google/protobuf/descriptor.proto
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 
@@ -21,118 +21,117 @@
     static DescriptorProtoFile() {
       byte[] descriptorData = global::System.Convert.FromBase64String(
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-            "bGVjdGlvbrACAQ=="));
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+            "CE5hbWVQYXJ0EhEKCW5hbWVfcGFydBgBIAIoCRIUCgxpc19leHRlbnNpb24Y", 
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+            "Z29vZ2xlLnByb3RvYnVmLlNvdXJjZUNvZGVJbmZvLkxvY2F0aW9uGoYBCghM", 
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+            "GAQgASgJEiEKGWxlYWRpbmdfZGV0YWNoZWRfY29tbWVudHMYBiADKAlCWwoT", 
+            "Y29tLmdvb2dsZS5wcm90b2J1ZkIQRGVzY3JpcHRvclByb3Rvc0gBWgpkZXNj", 
+            "cmlwdG9yogIDR1BCqgIaR29vZ2xlLlByb3RvYnVmLlJlZmxlY3Rpb26wAgE="));
       descriptor = pbr::FileDescriptor.InternalBuildGeneratedFileFrom(descriptorData,
           new pbr::FileDescriptor[] { },
           new pbr::GeneratedCodeInfo(null, new pbr::GeneratedCodeInfo[] {
diff --git a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
index bad58fa..45862af 100644
--- a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
@@ -300,8 +300,9 @@
                 if (dependencies[i].Name != proto.Dependency[i])
                 {
                     throw new DescriptorValidationException(result,
-                                                            "Dependencies passed to FileDescriptor.BuildFrom() don't match " +
-                                                            "those listed in the FileDescriptorProto.");
+                        "Dependencies passed to FileDescriptor.BuildFrom() don't match " +
+                        "those listed in the FileDescriptorProto. Expected: " +
+                        proto.Dependency[i] + " but was: " + dependencies[i].Name);
                 }
             }
 
diff --git a/src/google/protobuf/compiler/csharp/csharp_helpers.cc b/src/google/protobuf/compiler/csharp/csharp_helpers.cc
index d25dcba..333b491 100644
--- a/src/google/protobuf/compiler/csharp/csharp_helpers.cc
+++ b/src/google/protobuf/compiler/csharp/csharp_helpers.cc
@@ -118,6 +118,12 @@
 }
 
 std::string GetUmbrellaClassUnqualifiedName(const FileDescriptor* descriptor) {
+  // We manually rename Descriptor to DescriptorProtoFile to avoid collisions with
+  // the static Descriptor property. It would be nice to be able to do this with an
+  // option, but it would be rarely used.
+  if (IsDescriptorProto(descriptor)) {
+    return "DescriptorProtoFile";
+  }
   // umbrella_classname can no longer be set using message option.
   std::string proto_file = descriptor->name();
   int lastslash = proto_file.find_last_of("/");
diff --git a/src/google/protobuf/compiler/csharp/csharp_helpers.h b/src/google/protobuf/compiler/csharp/csharp_helpers.h
index 278e05f..4ed17a8 100644
--- a/src/google/protobuf/compiler/csharp/csharp_helpers.h
+++ b/src/google/protobuf/compiler/csharp/csharp_helpers.h
@@ -115,12 +115,7 @@
 // for use in the runtime. This is the only type which is allowed to use proto2 syntax,
 // and it generates internal classes.
 inline bool IsDescriptorProto(const FileDescriptor* descriptor) {
-  // TODO: Do this better! (Currently this depends on a hack in generate_protos.sh to rename
-  // the file...)
-  // We need to be able to detect the "normal" name as well, for times that we're just
-  // depending on descriptor.proto instead of generating it.
-  return descriptor->name() == "google/protobuf/descriptor_proto_file.proto"
-      || descriptor->name() == "google/protobuf/descriptor.proto";
+  return descriptor->name() == "google/protobuf/descriptor.proto";
 }
 
 inline bool IsWrapperType(const FieldDescriptor* descriptor) {