Move MessageWriter from snapshot files to dart_api_message files

runtime/vm/snapshot_test.cc

 // 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.
 
 #include "include/dart_debugger_api.h"
 #include "platform/assert.h"
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_api_message.h"
@@ skipping 101 matching lines @@
       break;
     default:
       EXPECT(false);
   }
 }
 
 
 static void CheckEncodeDecodeMessage(Dart_CObject* root) {
   // Encode and decode the message.
   uint8_t* buffer = NULL;
-  MessageWriter writer(&buffer, &malloc_allocator);
+  ApiMessageWriter writer(&buffer, &malloc_allocator);
   writer.WriteCMessage(root);
 
   Dart_CObject* new_root = DecodeMessage(buffer + Snapshot::kHeaderSize,
                                          writer.BytesWritten(),
                                          &zone_allocator);
 
   // Check that the two messages are the same.
   CompareDartCObjects(root, new_root);
 }
 
@@ skipping 891 matching lines @@
   }
   Dart_ShutdownIsolate();
   free(full_snapshot);
   free(script_snapshot);
 }
 
 
 TEST_CASE(IntArrayMessage) {
   Zone zone(Isolate::Current());
   uint8_t* buffer = NULL;
-  MessageWriter writer(&buffer, &zone_allocator);
+  ApiMessageWriter writer(&buffer, &zone_allocator);
 
   static const int kArrayLength = 2;
   intptr_t data[kArrayLength] = {1, 2};
   int len = kArrayLength;
   writer.WriteMessage(len, data);
 
   // Read object back from the snapshot into a C structure.
   ApiNativeScope scope;
   Dart_CObject* root = DecodeMessage(buffer + Snapshot::kHeaderSize,
                                      writer.BytesWritten(),
@@ skipping 487 matching lines @@
   EXPECT(Dart_ErrorHasException(result));
   EXPECT_SUBSTRING("Exception: nulltruefalse1234563.14[]\n",
                    Dart_GetError(result));
 
   Dart_ExitScope();
 }
 
 #endif  // defined(TARGET_ARCH_IA32) || defined(TARGET_ARCH_X64).
 
 }  // namespace dart