Fixes Android build by using C++ math header instead of C one.

runtime/platform/math.h

Index: runtime/platform/math.h
===================================================================
--- runtime/platform/math.h	(revision 0)
+++ runtime/platform/math.h	(revision 0)
@@ -0,0 +1,18 @@
+// Copyright (c) 2014, 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_MATH_H_
+#define PLATFORM_MATH_H_
+
+// We must take these math functions from the C++ header file as long as we
+// are using the STL. Otherwise the Android build will break due to confusion
+// between C++ and C headers when math.h is also included.
+#include <cmath>
+
+#define isinf(val) std::isinf(val)
+#define isnan(val) std::isnan(val)
+#define signbit(val) std::signbit(val)
+#define isfinite(val) std::isfinite(val)
+
+#endif  // PLATFORM_MATH_H_