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

runtime/vm/bigint_test.cc

 // Copyright (c) 2014, 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 "platform/assert.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
 static uword ZoneAllocator(intptr_t size) {
-  Zone* zone = Isolate::Current()->current_zone();
+  Zone* zone = Thread::Current()->zone();
   return zone->AllocUnsafe(size);
 }
 
 
 TEST_CASE(BigintSmi) {
   {
     const Smi& smi = Smi::Handle(Smi::New(5));
     const Bigint& bigint = Bigint::Handle(Bigint::NewFromInt64(smi.Value()));
     EXPECT_EQ(5, bigint.AsInt64Value());
     EXPECT(bigint.FitsIntoSmi());
@@ skipping 623 matching lines @@
 
   {
     const Bigint& bigint = Bigint::Handle(
         Bigint::NewFromCString("-1311768467463790320"));
     const char* str = bigint.ToHexCString(&ZoneAllocator);
     EXPECT_STREQ("-0x123456789ABCDEF0", str);
   }
 }
 
 }  // namespace dart