Make isolate.h usable without objects-inl.h header.

test/cctest/cctest.h

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 11 matching lines @@
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #ifndef CCTEST_H_
 #define CCTEST_H_
 
 #include "include/libplatform/libplatform.h"
+#include "src/isolate-inl.h"  // TODO(everyone): Make cctest IWYU.
 #include "src/objects-inl.h"  // TODO(everyone): Make cctest IWYU.
 #include "src/v8.h"
 
 #ifndef TEST
 #define TEST(Name)                                                             \
   static void Test##Name();                                                    \
   CcTest register_test_##Name(Test##Name, __FILE__, #Name, NULL, true, true);  \
   static void Test##Name()
 #endif
 
@@ skipping 558 matching lines @@
   HandleAndZoneScope() {}
 
   // Prefixing the below with main_ reduces a lot of naming clashes.
   i::Zone* main_zone() { return &main_zone_; }
 
  private:
   i::Zone main_zone_;
 };
 
 #endif  // ifndef CCTEST_H_