Make CTX allocatable by the register allocator.

runtime/vm/native_arguments.h

 // 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.
 
 #ifndef VM_NATIVE_ARGUMENTS_H_
 #define VM_NATIVE_ARGUMENTS_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 #include "vm/handles_impl.h"
@@ skipping 83 matching lines @@
 
   int NativeArgCount() const {
     int function_bits = FunctionBits::decode(argc_tag_);
     return ArgCount() - NumHiddenArgs(function_bits);
   }
 
   RawObject* NativeArg0() const {
     int function_bits = FunctionBits::decode(argc_tag_);
     if (function_bits == (kClosureFunctionBit | kInstanceFunctionBit)) {
       // Retrieve the receiver from the context.
-      const Context& context = Context::Handle(isolate_->top_context());
+      const Object& closure = Object::Handle(ArgAt(0));
+      const Context& context =
+          Context::Handle(Closure::context(Instance::Cast(closure)));
       return context.At(0);
     }
     return ArgAt(NumHiddenArgs(function_bits));
   }
 
   RawObject* NativeArgAt(int index) const {
     ASSERT((index >= 0) && (index < NativeArgCount()));
     if (index == 0) {
       return NativeArg0();
     }
@@ skipping 102 matching lines @@
 
   Isolate* isolate_;  // Current isolate pointer.
   int argc_tag_;  // Encodes argument count and invoked native call type.
   RawObject*(*argv_)[];  // Pointer to an array of arguments to runtime call.
   RawObject** retval_;  // Pointer to the return value area.
 };
 
 }  // namespace dart
 
 #endif  // VM_NATIVE_ARGUMENTS_H_