1. Add flag --trace-api to trace API function invocation

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 30 matching lines @@
   uword current_sp = func();                                                   \
   ASSERT(Utils::IsAligned(current_sp, OS::ActivationFrameAlignment()));        \
 }
 #endif
 
 #define VERIFY_ON_TRANSITION                                                   \
   if (FLAG_verify_on_transition) {                                             \
     VerifyPointersVisitor::VerifyPointers();                                   \
     Isolate::Current()->heap()->Verify();                                      \
   }
-#define TRACE_NATIVES(name)                                                    \
+#define TRACE_NATIVE_CALL(format, name)                                        \
   if (FLAG_trace_natives) {                                                    \
-    OS::Print("Calling native: %s\n", name);                                   \
+    OS::Print("Calling native: " format "\n", name);                           \
   }
 #define DEOPTIMIZE_ALOT                                                        \
   if (FLAG_deoptimize_alot) {                                                  \
     DeoptimizeAll();                                                           \
   }
 
 #else
 
 #define CHECK_STACK_ALIGNMENT { }
 #define VERIFY_ON_TRANSITION { }
-#define TRACE_NATIVES(name) { }
+#define TRACE_NATIVE_CALL(format, name) { }
 #define DEOPTIMIZE_ALOT { }
 
 #endif
 
 
 // Class NativeArguments is used to access arguments passed in from
 // generated dart code to a runtime function or a dart library native
 // function. It is also used to set the return value if any at the slot
 // reserved for return values.
 // All runtime function/dart library native functions have the
@@ skipping 116 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_