Updated VM stacktrace support (20898) with strndup impl.

runtime/platform/c99_support_win.h

 // Copyright (c) 2011, 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 PLATFORM_C99_SUPPORT_WIN_H_
 #define PLATFORM_C99_SUPPORT_WIN_H_
 
 // Visual C++ is missing a bunch of C99 math macros and
 // functions. Define them here.
 
 #include <float.h>
+#include <string.h>
 
 static const unsigned __int64 kQuietNaNMask =
     static_cast<unsigned __int64>(0xfff) << 51;
 
 
 #ifndef va_copy
 #define va_copy(dst, src) (memmove(&(dst), &(src), sizeof(dst)))
 #endif  /* va_copy */
 
 
@@ skipping 35 matching lines @@
 
   if (fractpart >= 0.5) {
     return intpart + 1;
   } else if (fractpart > -0.5) {
     return intpart;
   } else {
     return intpart - 1;
   }
 }
 
+// size_t used to match function signature on other platforms.
+static inline char* strndup(const char* s, size_t n) {
+  size_t len = strlen(s);
+  if (n < len) {
+    len = n;
+  }
+  char* result = reinterpret_cast<char*>(malloc(len + 1));
+  if (!result) {
+    return NULL;
+  }
+  result[len] = '\0';
+  return reinterpret_cast<char*>(memcpy(result, s, len));
+}
+
 #endif  // PLATFORM_C99_SUPPORT_WIN_H_