pkg/unittest: Fix warnings reported by analyzer

test/io.dart

 // Copyright (c) 2015, 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 unittest.test.io;
 
 import 'dart:io';
 import 'dart:mirrors';
 
 import 'package:path/path.dart' as p;
-import 'package:stack_trace/stack_trace.dart';
 
 /// The root directory of the `unittest` package.
 final String packageDir = _computePackageDir();
 String _computePackageDir() =>
     p.dirname(p.dirname(_libraryPath(#unittest.test.io)));
 
 /// Runs the unittest executable with the package root set properly.
 ProcessResult runUnittest(List<String> args, {String workingDirectory,
     Map<String, String> environment}) {
   var allArgs = [
@@ skipping 22 matching lines @@
 /// Returns the path to the library named [libraryName].
 ///
 /// The library name must be globally unique, or the wrong library path may be
 /// returned.
 String _libraryPath(Symbol libraryName) {
   var lib = currentMirrorSystem().findLibrary(libraryName);
   return p.fromUri(lib.uri);
 }