remove docgen source and targets from build

sdk/bin/dartdocgen

-#!/bin/bash
-# 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 "${PROG_NAME%/*}" ; pwd -P)"
-
-SDK_DIR="$(cd "${BIN_DIR}/.." ; pwd -P)"
-
-unset SNAPSHOT
-
-SNAPSHOT="$BIN_DIR/snapshots/utils_wrapper.dart.snapshot"
-
-if test -f "$SNAPSHOT"; then
-  exec "$BIN_DIR"/dart \
-      "--package-root=$BIN_DIR/../packages/" "--old_gen_heap_size=1024" \
-      "$SNAPSHOT" \
-       docgen "--sdk=$SDK_DIR" "$@"
-else
-  exec "$BIN_DIR"/dart \
-      "--package-root=$BIN_DIR/../packages/" "--old_gen_heap_size=1024" \
-      "$BIN_DIR/../../pkg/docgen/bin/docgen.dart" "--sdk=$SDK_DIR" "$@"
-fi