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

runtime/vm/stack_frame_test.cc

Index: runtime/vm/stack_frame_test.cc
diff --git a/runtime/vm/stack_frame_test.cc b/runtime/vm/stack_frame_test.cc
index fd728776f899883b8d3a7699e9a319b302d9ea1b..426ede7941ffca93ce17712d3daea40db621e633 100644
--- a/runtime/vm/stack_frame_test.cc
+++ b/runtime/vm/stack_frame_test.cc
@@ -97,13 +97,12 @@ void FUNCTION_NAME(StackFrame_validateFrame)(Dart_NativeArguments args) {
       }
       const char* name = function.ToFullyQualifiedCString();
       // Currently all unit tests are loaded as being part of dart:core-lib.
-      Isolate* isolate = Isolate::Current();
       String& url = String::Handle(String::New(TestCase::url()));
       const Library& lib = Library::Handle(Library::LookupLibrary(url));
       ASSERT(!lib.IsNull());
       const char* lib_name = String::Handle(lib.url()).ToCString();
       intptr_t length = OS::SNPrint(NULL, 0, "%s_%s", lib_name, expected_name);
-      char* full_name = isolate->current_zone()->Alloc<char>(length + 1);
+      char* full_name = Thread::Current()->zone()->Alloc<char>(length + 1);
       ASSERT(full_name != NULL);
       OS::SNPrint(full_name, (length + 1), "%s_%s", lib_name, expected_name);
       if (strcmp(full_name, name) != 0) {