Fix floating point issues on Windows.

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.
 
@@ skipping 30 matching lines @@
 
 static inline double trunc(double x) {
   if (x < 0) {
     return ceil(x);
   } else {
     return floor(x);
   }
 }
 
 static inline double round(double x) {
-  if (x < 0) {
-    return ceil(x - 0.5);
+  if (!_finite(x)) {
+    return x;
+  }
+
+  double intpart;
+  double fractpart = modf(x, &intpart);
+
+  if (fractpart >= 0.5) {
+    return intpart + 1;
+  } else if (fractpart > -0.5) {
+    return intpart;
   } else {
-    return floor(x + 0.5);
+    return intpart - 1;
   }
 }
 
 #endif  // PLATFORM_C99_SUPPORT_WIN_H_