Fixed snapshot name in dart2js.bat, dartdoc.bat invocation scripts.

utils/compiler/build_helper.dart

Index: utils/compiler/build_helper.dart
diff --git a/utils/compiler/build_helper.dart b/utils/compiler/build_helper.dart
index fa2ec0c3687c8bafabf2d75841764578c66e5482..8b7363b87f4e3e7082417f631b80e53742232ba6 100644
--- a/utils/compiler/build_helper.dart
+++ b/utils/compiler/build_helper.dart
@@ -143,8 +143,12 @@ REM Does the path have a trailing slash? If so, remove it.
 if %SCRIPTPATH:~-1%==\ set SCRIPTPATH=%SCRIPTPATH:~0,-1%
 
 set arguments=%*
-set SNAPSHOTNAME=%SCRIPTPATH%dart2js.snapshot
-if exist %SNAPSHOTNAME% set SNAPSHOT=--use_script_snapshot=%SNAPSHOTNAME%
+set SNAPSHOT=
+set SNAPSHOTNAME=%SCRIPTPATH%${pathWin}.snapshot
+if exist %SNAPSHOTNAME% (
+  echo Using snapshot "%SNAPSHOTNAME%" 1>&2
+  set SNAPSHOT=--use_script_snapshot=%SNAPSHOTNAME%
+)
 
 "%SCRIPTPATH%\dart.exe"$options %SNAPSHOT% "%SCRIPTPATH%$pathWin" %arguments%
 '''.replaceAll('\n', '\r\n')];