Automatically pass SDK into docgen.dart, and use the SDK for running

sdk/bin/docgen

 #!/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
-  # TODO(ahe): Remove the following line when we are relatively sure it works.
-  echo Using snapshot $SNAPSHOT 1>&2
   exec "$BIN_DIR"/dart \
-      "--package-root=$BIN_DIR/../packages/" $SNAPSHOT docgen "$@"
+      "--package-root=$BIN_DIR/../packages/" $SNAPSHOT \
+      docgen "--sdk=$SDK_DIR" "$@"
 else
   exec "$BIN_DIR"/dart \
       "--package-root=$BIN_DIR/../packages/" \
-      "$BIN_DIR/../pkg/docgen/bin/docgen.dart" "$@"
+      "$BIN_DIR/../../pkg/docgen/bin/docgen.dart" "--sdk=$SDK_DIR" "$@"
 fi