dart2js: don't add a trailing slash when reading --packages

pkg/compiler/lib/src/dart2js.dart

 // Copyright (c) 2013, 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.
 
 library dart2js.cmdline;
 
 import 'dart:async'
     show Future, EventSink;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io'
@@ skipping 48 matching lines @@
   // m[0] is the entire match (which will be equal to argument). m[1]
   // is something like "-o" or "--out=", and m[2] is the parameter.
   Match m = new RegExp('^(-[a-z]|--.+=)(.*)').firstMatch(argument);
   if (m == null) {
     if (isOptionalArgument) return null;
     helpAndFail('Unknown option "$argument".');
   }
   return m[2];
 }
 
-String extractPath(String argument) {
+String extractPath(String argument, {bool isDirectory: true}) {
   String path = nativeToUriPath(extractParameter(argument));
-  return path.endsWith("/") ? path : "$path/";
+  return !path.endsWith("/") && isDirectory ? "$path/" : path;
 }
 
 void parseCommandLine(List<OptionHandler> handlers, List<String> argv) {
   // TODO(ahe): Use ../../args/args.dart for parsing options instead.
   var patterns = <String>[];
   for (OptionHandler handler in handlers) {
     patterns.add(handler.pattern);
   }
   var pattern = new RegExp('^(${patterns.join(")\$|^(")})\$');
 
@@ skipping 54 matching lines @@
 
   setLibraryRoot(String argument) {
     libraryRoot = currentDirectory.resolve(extractPath(argument));
   }
 
   setPackageRoot(String argument) {
     packageRoot = currentDirectory.resolve(extractPath(argument));
   }
 
   setPackageConfig(String argument) {
-    packageConfig = currentDirectory.resolve(extractPath(argument));
+    packageConfig =
+        currentDirectory.resolve(extractPath(argument, isDirectory: false));
   }
 
   setOutput(Iterator<String> arguments) {
     optionsImplyCompilation.add(arguments.current);
     String path;
     if (arguments.current == '-o') {
       if (!arguments.moveNext()) {
         helpAndFail('Error: Missing file after -o option.');
       }
       path = arguments.current;
@@ skipping 609 matching lines @@
       } else if (exitCode == 253) {
         print(">>> TEST CRASH");
       } else {
         print(">>> TEST FAIL");
       }
       stderr.writeln(">>> EOF STDERR");
       subscription.resume();
     });
   });
 }