Use builtin fmod for double modulo operation.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 17487)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -2632,6 +2632,7 @@
       ASSERT(!CPUFeatures::double_truncate_round_supported());
       return 1;
     }
+    case MethodRecognizer::kDoubleMod:
     case MethodRecognizer::kDoublePow:
       return 2;
     default:
@@ -2648,6 +2649,10 @@
     "libc_pow", reinterpret_cast<RuntimeFunction>(
         static_cast<BinaryMathCFunction>(&pow)), 0, true);
 
+extern const RuntimeEntry kModRuntimeEntry(
+    "libc_fmod", reinterpret_cast<RuntimeFunction>(
+        static_cast<BinaryMathCFunction>(&fmod)), 0, true);
+
 extern const RuntimeEntry kFloorRuntimeEntry(
     "libc_floor", reinterpret_cast<RuntimeFunction>(
         static_cast<UnaryMathCFunction>(&floor)), 0, true);
@@ -2677,6 +2682,8 @@
       return kCeilRuntimeEntry;
     case MethodRecognizer::kDoublePow:
       return kPowRuntimeEntry;
+    case MethodRecognizer::kDoubleMod:
+      return kModRuntimeEntry;
     default:
       UNREACHABLE();
   }