Fix background compilation issues when creating snapshots.

runtime/vm/compiler.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/compiler.h"
 
 #include "vm/assembler.h"
 
 #include "vm/ast_printer.h"
 #include "vm/block_scheduler.h"
@@ skipping 1760 matching lines @@
   Isolate::Current()->set_background_compiler(NULL);
 }
 
 
 void BackgroundCompiler::EnsureInit(Thread* thread) {
   ASSERT(thread->IsMutatorThread());
   // Finalize NoSuchMethodError, _Mint; occasionally needed in optimized
   // compilation.
   Class& cls = Class::Handle(thread->zone(),
       Library::LookupCoreClass(Symbols::NoSuchMethodError()));
+  ASSERT(!cls.IsNull());
   Error& error = Error::Handle(thread->zone(),
       cls.EnsureIsFinalized(thread));
   ASSERT(error.IsNull());
   cls = Library::LookupCoreClass(Symbols::_Mint());
+  ASSERT(!cls.IsNull());
   error = cls.EnsureIsFinalized(thread);
   ASSERT(error.IsNull());
 
   bool start_task = false;
   Isolate* isolate = thread->isolate();
   {
     MutexLocker ml(isolate->mutex());
     if (isolate->background_compiler() == NULL) {
       BackgroundCompiler* task = new BackgroundCompiler(isolate);
       isolate->set_background_compiler(task);
@@ skipping 117 matching lines @@
 }
 
 
 void BackgroundCompiler::EnsureInit(Thread* thread) {
   UNREACHABLE();
 }
 
 #endif  // DART_PRECOMPILED_RUNTIME
 
 }  // namespace dart