Include what you use for the preparser.

src/fixed-dtoa.h

-// Copyright 2010 the V8 project authors. All rights reserved.
+// Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
 //       with the distribution.
@@ skipping 12 matching lines @@
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #ifndef V8_FIXED_DTOA_H_
 #define V8_FIXED_DTOA_H_
 
 namespace v8 {
 namespace internal {
 
+template <typename T> class Vector;
+
 // Produces digits necessary to print a given number with
 // 'fractional_count' digits after the decimal point.
 // The buffer must be big enough to hold the result plus one terminating null
 // character.
 //
 // The produced digits might be too short in which case the caller has to fill
 // the gaps with '0's.
 // Example: FastFixedDtoa(0.001, 5, ...) is allowed to return buffer = "1", and
 // decimal_point = -2.
 // Halfway cases are rounded towards +/-Infinity (away from 0). The call
 // FastFixedDtoa(0.15, 2, ...) thus returns buffer = "2", decimal_point = 0.
 // The returned buffer may contain digits that would be truncated from the
 // shortest representation of the input.
 //
 // This method only works for some parameters. If it can't handle the input it
 // returns false. The output is null-terminated when the function succeeds.
 bool FastFixedDtoa(double v, int fractional_count,
                    Vector<char> buffer, int* length, int* decimal_point);
 
 } }  // namespace v8::internal
 
 #endif  // V8_FIXED_DTOA_H_