Use string.Concat instead of String.Concat to avoid potential import problems,

and fix number of times the final base64 chunk is emitted.

src/ProtoGen/UmbrellaClassGenerator.cs

@@ -143,7 +143,6 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("#region Designer generated code");
 
             writer.WriteLine();
-            writer.WriteLine("using System;");
             writer.WriteLine("using pb = global::Google.ProtocolBuffers;");
             writer.WriteLine("using pbc = global::Google.ProtocolBuffers.Collections;");
             writer.WriteLine("using pbd = global::Google.ProtocolBuffers.Descriptors;");
@@ -207,18 +206,19 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("byte[] descriptorData = global::System.Convert.FromBase64String(");
             writer.Indent();
             writer.Indent();
-            writer.WriteLine("String.Concat(");
+            writer.WriteLine("string.Concat(");
+            writer.Indent();
             // TODO(jonskeet): Consider a C#-escaping format here instead of just Base64.
             byte[] bytes = Descriptor.Proto.ToByteArray();
             string base64 = Convert.ToBase64String(bytes);
 
             while (base64.Length > 60)
             {
-                writer.WriteLine("\"{0}\" , ", base64.Substring(0, 60));
+                writer.WriteLine("\"{0}\", ", base64.Substring(0, 60));
                 base64 = base64.Substring(60);
             }
-            writer.WriteLine("\"{0}\") );", base64);
-            writer.WriteLine("\"{0}\");", base64);
+            writer.Outdent();
+            writer.WriteLine("\"{0}\"));", base64);
             writer.Outdent();
             writer.Outdent();
             writer.WriteLine(