Include what you use for the preparser.

src/dtoa.h

-// Copyright 2011 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_DTOA_H_
 #define V8_DTOA_H_
 
 namespace v8 {
 namespace internal {
 
+template <typename T> class Vector;
+
 enum DtoaMode {
   // Return the shortest correct representation.
   // For example the output of 0.299999999999999988897 is (the less accurate but
   // correct) 0.3.
   DTOA_SHORTEST,
   // Return a fixed number of digits after the decimal point.
   // For instance fixed(0.1, 4) becomes 0.1000
   // If the input number is big, the output will be big.
   DTOA_FIXED,
   // Return a fixed number of digits, no matter what the exponent is.
@@ skipping 32 matching lines @@
 // 'DoubleToAscii' expects the given buffer to be big enough to hold all digits
 // and a terminating null-character. In SHORTEST-mode it expects a buffer of
 // at least kBase10MaximalLength + 1. Otherwise, the size of the output is
 // limited to requested_digits digits plus the null terminator.
 void DoubleToAscii(double v, DtoaMode mode, int requested_digits,
                    Vector<char> buffer, int* sign, int* length, int* point);
 
 } }  // namespace v8::internal
 
 #endif  // V8_DTOA_H_