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

runtime/vm/object_test.cc

Index: runtime/vm/object_test.cc
===================================================================
--- runtime/vm/object_test.cc	(revision 37827)
+++ runtime/vm/object_test.cc	(working copy)
@@ -2,7 +2,6 @@
 // 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.
 
-// TODO(zra): Remove when tests are ready to enable.
 #include "platform/globals.h"
 
 #include "vm/assembler.h"
@@ -527,7 +526,7 @@
     EXPECT(!dbl1.OperatorEquals(Smi::Handle(Smi::New(3))));
     EXPECT(!dbl1.OperatorEquals(Double::Handle()));
     const Double& nan0 = Double::Handle(Double::New(NAN));
-    EXPECT(isnan(nan0.value()));
+    EXPECT(std::isnan(nan0.value()));
     EXPECT(nan0.IsIdenticalTo(nan0));
     EXPECT(nan0.CanonicalizeEquals(nan0));
     EXPECT(!nan0.OperatorEquals(nan0));
@@ -535,8 +534,8 @@
         Double::New(bit_cast<double>(kMaxUint64 - 0)));
     const Double& nan2 = Double::Handle(
         Double::New(bit_cast<double>(kMaxUint64 - 1)));
-    EXPECT(isnan(nan1.value()));
-    EXPECT(isnan(nan2.value()));
+    EXPECT(std::isnan(nan1.value()));
+    EXPECT(std::isnan(nan2.value()));
     EXPECT(!nan1.IsIdenticalTo(nan2));
     EXPECT(!nan1.CanonicalizeEquals(nan2));
     EXPECT(!nan1.OperatorEquals(nan2));