Slight refactoring of Extensions to support lookup by name, added compatibility tests for text and binary formats.
diff --git a/src/ProtocolBuffers.Test/TextFormatTest.cs b/src/ProtocolBuffers.Test/TextFormatTest.cs
index 76b8920..49dac25 100644
--- a/src/ProtocolBuffers.Test/TextFormatTest.cs
+++ b/src/ProtocolBuffers.Test/TextFormatTest.cs
@@ -404,8 +404,8 @@
"1:2: Extension \"nosuchext\" not found in the ExtensionRegistry.",
"[nosuchext]: 123");
AssertParseError(
- "1:20: Extension \"protobuf_unittest.optional_int32_extension\" does " +
- "not extend message type \"protobuf_unittest.TestAllTypes\".",
+ "1:20: Extension \"protobuf_unittest.optional_int32_extension\" " +
+ "not found in the ExtensionRegistry.",
"[protobuf_unittest.optional_int32_extension]: 123");
AssertParseError(
"1:1: Message type \"protobuf_unittest.TestAllTypes\" has no field " +