Add a snapshot for docgen and use it in the build

sdk/bin/docgen

 #!/bin/bash
-# Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+# Copyright (c) 2014, 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.
 
 function follow_links() {
   file="$1"
   while [ -h "$file" ]; do
     # On Mac OS, readlink -f doesn't work.
     file="$(readlink "$file")"
   done
   echo "$file"
 }
 
 # Unlike $0, $BASH_SOURCE points to the absolute path of this file.
 PROG_NAME="$(follow_links "$BASH_SOURCE")"
 
 # Handle the case where dart-sdk/bin has been symlinked to.
 BIN_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
 
-unset COLORS
-if test -t 1; then
-  # Stdout is a terminal.
-  if test 8 -le `tput colors`; then
-    # Stdout has at least 8 colors, so enable colors.
-    COLORS="--enable-diagnostic-colors"
-  fi
-fi
-
 unset SNAPSHOT
 
 SNAPSHOT="$BIN_DIR/snapshots/utils_wrapper.dart.snapshot"
 
 if test -f $SNAPSHOT; then
   # TODO(ahe): Remove the following line when we are relatively sure it works.
   echo Using snapshot $SNAPSHOT 1>&2
-  exec "$BIN_DIR"/dart --heap_growth_rate=32 \
-      "--package-root=$BIN_DIR/../packages/" $SNAPSHOT dartdoc $COLORS \
-      "--package-root=$BIN_DIR/../packages/" "--library-root=$BIN_DIR/.." "$@"
+  exec "$BIN_DIR"/dart \
+      "--package-root=$BIN_DIR/../packages/" $SNAPSHOT docgen "$@"
 else
-  exec "$BIN_DIR"/dart --heap_growth_rate=32 \
+  exec "$BIN_DIR"/dart \
       "--package-root=$BIN_DIR/../packages/" \
-      "$BIN_DIR/../lib/_internal/dartdoc/bin/dartdoc.dart" $COLORS "$@"
+      "$BIN_DIR/../pkg/docgen/bin/docgen.dart" "$@"
 fi