A patch class must be given the same name as the class it is patching, otherwise

runtime/vm/parser.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/parser.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
@@ skipping 3222 matching lines @@
   } else {
     if (!obj.IsClass()) {
       ErrorMsg(classname_pos, "'%s' is already defined",
                class_name.ToCString());
     }
     cls ^= obj.raw();
     if (is_patch) {
       // Preserve and reuse the original type parameters and bounds since the
       // ones defined in the patch class will not be finalized.
       orig_type_parameters = cls.type_parameters();
-      String& patch = String::Handle(
-          String::Concat(Symbols::PatchSpace(), class_name));
-      patch = Symbols::New(patch);
-      cls = Class::New(patch, script_, classname_pos);
+      // A patch class must be given the same name as the class it is patching,
+      // otherwise the generic signature classes it defines will not match the
+      // patched generic signature classes. Therefore, new signature classes
+      // will be introduced and the original ones will not get finalized.
+      cls = Class::New(class_name, script_, classname_pos);
       cls.set_library(library_);
     } else {
       // Not patching a class, but it has been found. This must be one of the
       // pre-registered classes from object.cc or a duplicate definition.
       if (cls.functions() != Object::empty_array().raw()) {
         ErrorMsg(classname_pos, "class '%s' is already defined",
                  class_name.ToCString());
       }
       // Pre-registered classes need their scripts connected at this time.
       cls.set_script(script_);
@@ skipping 6660 matching lines @@
 void Parser::SkipQualIdent() {
   ASSERT(IsIdentifier());
   ConsumeToken();
   if (CurrentToken() == Token::kPERIOD) {
     ConsumeToken();  // Consume the kPERIOD token.
     ExpectIdentifier("identifier expected after '.'");
   }
 }
 
 }  // namespace dart