Add Log::VPrint and remove duplicated code.

runtime/vm/log.h

Index: runtime/vm/log.h
diff --git a/runtime/vm/log.h b/runtime/vm/log.h
index 78f062bdf5913f5af52152e3e9f4205e2cef2f9c..f4c58e964cffb4b30c51dbf1a0186b8ab32d4d82 100644
--- a/runtime/vm/log.h
+++ b/runtime/vm/log.h
@@ -23,6 +23,9 @@ class Thread;
     Isolate::Current()->Log()->Print(format, ##__VA_ARGS__)
 #endif
 
+#define ISL_VPrint(format, args) \
+    Isolate::Current()->Log()->VPrint(format, args)
+
 typedef void (*LogPrinter)(const char* str, ...);
 
 class Log {
@@ -32,6 +35,8 @@ class Log {
   // Append a formatted string to the log.
   void Print(const char* format, ...) PRINTF_ATTRIBUTE(2, 3);
 
+  void VPrint(const char* format, va_list args);
+
   // Flush and truncate the log. The log is flushed starting at cursor
   // and truncated to cursor afterwards.
   void Flush(const intptr_t cursor = 0);