Migrate most uses of Isolate::current_zone to Thread::zone.

runtime/vm/coverage.cc

 // Copyright (c) 2013, 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/coverage.h"
 
 #include "include/dart_api.h"
 
 #include "vm/compiler.h"
 #include "vm/isolate.h"
@@ skipping 234 matching lines @@
     return;
   }
 
   JSONStream stream;
   PrintJSON(isolate, &stream, NULL, false);
 
   const char* format = "%s/dart-cov-%" Pd "-%" Pd ".json";
   intptr_t pid = OS::ProcessId();
   intptr_t len = OS::SNPrint(NULL, 0, format,
                              FLAG_coverage_dir, pid, isolate->main_port());
-  char* filename = Isolate::Current()->current_zone()->Alloc<char>(len + 1);
+  char* filename = Thread::Current()->zone()->Alloc<char>(len + 1);
   OS::SNPrint(filename, len + 1, format,
               FLAG_coverage_dir, pid, isolate->main_port());
   void* file = (*file_open)(filename, true);
   if (file == NULL) {
     OS::Print("Failed to write coverage file: %s\n", filename);
     return;
   }
   (*file_write)(stream.buffer()->buf(), stream.buffer()->length(), file);
   (*file_close)(file);
 }
@@ skipping 22 matching lines @@
         cls = it.GetNextClass();
         ASSERT(!cls.IsNull());
         PrintClass(lib, cls, jsarr, filter, as_call_sites);
       }
     }
   }
 }
 
 
 }  // namespace dart