Implement Double.toStringAsFixed.

runtime/vm/double_conversion.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 VM_DOUBLE_CONVERSION_H_
+#define VM_DOUBLE_CONVERSION_H_
+
+#include "vm/globals.h"
+#include "vm/object.h"
+
+namespace dart {
+
+bool DoubleToString(double d, String& result);
+bool DoubleToStringAsFixed(double d, int fraction_digits, String& result);
+bool DoubleToStringAsExponential(double d, int fraction_digits, String& result);
+bool DoubleToStringAsPrecision(double d, int precision, String& result);
+
+}  // namespace dart
+
+#endif  // VM_DOUBLE_CONVERSION_H_