Make the "sticky error" an Error instead of a String.

runtime/vm/object.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/object.h"
 
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/bigint_operations.h"
 #include "vm/bootstrap.h"
@@ skipping 3313 matching lines @@
     const Script& script = Script::Handle(owner_class.script());
     signature_class = Class::NewSignatureClass(signature,
                                                closure_function,
                                                script);
     library.AddClass(signature_class);
   } else {
     closure_function.set_signature_class(signature_class);
   }
   const Type& signature_type = Type::Handle(signature_class.SignatureType());
   if (!signature_type.IsFinalized()) {
-    String& errmsg = String::Handle();
+    Error& error = Error::Handle();
     ClassFinalizer::FinalizeAndCanonicalizeType(signature_class,
                                                 signature_type,
-                                                &errmsg);
-    ASSERT(errmsg.IsNull());
+                                                &error);
+    ASSERT(error.IsNull());
   }
   ASSERT(closure_function.signature_class() == signature_class.raw());
   set_implicit_closure_function(closure_function);
   ASSERT(closure_function.IsImplicitClosureFunction());
   return closure_function.raw();
 }
 
 
 template<typename T>
 static RawArray* NewArray(const GrowableArray<T*>& objs) {
@@ skipping 4260 matching lines @@
   const String& str = String::Handle(pattern());
   const char* format = "JSRegExp: pattern=%s flags=%s";
   intptr_t len = OS::SNPrint(NULL, 0, format, str.ToCString(), Flags());
   char* chars = reinterpret_cast<char*>(
       Isolate::Current()->current_zone()->Allocate(len + 1));
   OS::SNPrint(chars, (len + 1), format, str.ToCString(), Flags());
   return chars;
 }
 
 }  // namespace dart