Fix tools/full-coverage.dart script and add standalone test

tools/full-coverage.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.
 
 import "dart:async";
 import "dart:convert";
 import "dart:io";
 import "dart:isolate";
 
 import "package:args/args.dart";
 import "package:path/path.dart";
 
 /// [Environment] stores gathered arguments information.
 class Environment {
   String sdkRoot;
   String pkgRoot;
   var input;
   var output;
   int workers;
-  bool prettyPrint;
-  bool lcov;
+  bool prettyPrint = false;
+  bool lcov = false;
   bool expectMarkers;
   bool verbose;
 }
 
 /// [Resolver] resolves imports with respect to a given environment.
 class Resolver {
   static const DART_PREFIX = "dart:";
   static const PACKAGE_PREFIX = "package:";
   static const FILE_PREFIX = "file://";
   static const HTTP_PREFIX = "http://";
@@ skipping 366 matching lines @@
 }
 
 /// Checks the validity of the provided arguments. Does not initialize actual
 /// processing.
 parseArgs(List<String> arguments) {
   var parser = new ArgParser();
 
   parser.addOption("sdk-root", abbr: "s",
                    help: "path to the SDK root");
   parser.addOption("package-root", abbr: "p",
-                   help: "path to the package root");
+                   help: "override path to the package root "
+                         "(default: inherited from dart)");
   parser.addOption("in", abbr: "i",
                    help: "input(s): may be file or directory");
   parser.addOption("out", abbr: "o",
                    help: "output: may be file or stdout",
                    defaultsTo: "stdout");
   parser.addOption("workers", abbr: "j",
                    help: "number of workers",
                    defaultsTo: "1");
   parser.addFlag("pretty-print", abbr: "r",
                  help: "convert coverage data to pretty print format",
@@ skipping 39 matching lines @@
     fail("Provided SDK root '${args["sdk-root"]}' is not a valid SDK "
          "top-level directory");
   }
 
   env.pkgRoot = args["package-root"];
   if (env.pkgRoot != null) {
     env.pkgRoot = absolute(normalize(args["package-root"]));
     if (!FileSystemEntity.isDirectorySync(env.pkgRoot)) {
       fail("Provided package root '${args["package-root"]}' is not directory.");
     }
+  } else {
+    env.pkgRoot = Platform.packageRoot;
   }
 
   if (args["in"] == null) {
     fail("No input files given.");
   } else {
     env.input = absolute(normalize(args["in"]));
     if (!FileSystemEntity.isDirectorySync(env.input) &&
         !FileSystemEntity.isFileSync(env.input)) {
       fail("Provided input '${args["in"]}' is neither a directory, nor a file.");
     }
@@ skipping 15 matching lines @@
   }
 
   try {
     env.workers = int.parse("${args["workers"]}");
   } catch (e) {
     fail("Invalid worker count: $e");
   }
 
   env.verbose = args["verbose"];
 }