Fixes a couple problems with GC of unoptimized code.

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 698 matching lines @@
 
 
 static RawError* CompileFunctionHelper(const Function& function,
                                        bool optimized,
                                        intptr_t osr_id) {
   Isolate* isolate = Isolate::Current();
   StackZone zone(isolate);
   LongJump* base = isolate->long_jump_base();
   LongJump jump;
   isolate->set_long_jump_base(&jump);
+  // Make sure unoptimized code is not collected while we are compiling.
+  const Code& unoptimized_code = Code::ZoneHandle(function.unoptimized_code());
   // Skips parsing if we need to only install unoptimized code.
-  if (!optimized && !Code::Handle(function.unoptimized_code()).IsNull()) {
+  if (!optimized && !unoptimized_code.IsNull()) {
     InstallUnoptimizedCode(function);
     isolate->set_long_jump_base(base);
     return Error::null();
   }
   if (setjmp(*jump.Set()) == 0) {
     TIMERSCOPE(time_compilation);
     Timer per_compile_timer(FLAG_trace_compiler, "Compilation time");
     per_compile_timer.Start();
     ParsedFunction* parsed_function =
         new ParsedFunction(Function::ZoneHandle(function.raw()));
@@ skipping 197 matching lines @@
       Object::Handle(isolate->object_store()->sticky_error());
     isolate->object_store()->clear_sticky_error();
     isolate->set_long_jump_base(base);
     return result.raw();
   }
   UNREACHABLE();
   return Object::null();
 }
 
 }  // namespace dart