Tell the user if the binstub directory is not on their path.

sdk/lib/_internal/pub_generated/test/test_pub.dart

 library test_pub;
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:math';
 import 'package:http/testing.dart';
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_server.dart';
 import 'package:scheduled_test/scheduled_stream.dart';
@@ skipping 224 matching lines @@
   schedule(
       () => renameDir(p.join(sandboxDir, from), p.join(sandboxDir, to)),
       'renaming $from to $to');
 }
 void scheduleSymlink(String target, String symlink) {
   schedule(
       () => createSymlink(p.join(sandboxDir, target), p.join(sandboxDir, symlink)),
       'symlinking $target to $symlink');
 }
 void schedulePub({List args, output, error, outputJson, int exitCode:
-    exit_codes.SUCCESS}) {
+    exit_codes.SUCCESS, Map<String, String> environment}) {
   assert(output == null || outputJson == null);
-  var pub = startPub(args: args);
+  var pub = startPub(args: args, environment: environment);
   pub.shouldExit(exitCode);
   var failures = [];
   var stderr;
   expect(
       Future.wait(
           [pub.stdoutStream().toList(), pub.stderrStream().toList()]).then((results) {
     var stdout = results[0].join("\n");
     stderr = results[1].join("\n");
     if (outputJson == null) {
       _validateOutput(failures, 'stdout', output, stdout);
@@ skipping 28 matching lines @@
 Map getPubTestEnvironment([String tokenEndpoint]) {
   var environment = {};
   environment['_PUB_TESTING'] = 'true';
   environment['PUB_CACHE'] = _pathInSandbox(cachePath);
   environment['_PUB_TEST_SDK_VERSION'] = "0.1.2+3";
   if (tokenEndpoint != null) {
     environment['_PUB_TEST_TOKEN_ENDPOINT'] = tokenEndpoint.toString();
   }
   return environment;
 }
-ScheduledProcess startPub({List args, Future<String> tokenEndpoint}) {
+ScheduledProcess startPub({List args, Future<String> tokenEndpoint, Map<String,
+    String> environment}) {
   ensureDir(_pathInSandbox(appPath));
   var dartBin = Platform.executable;
   if (dartBin.contains(Platform.pathSeparator)) {
     dartBin = p.absolute(dartBin);
   }
   var pubPath = p.join(p.dirname(dartBin), 'snapshots/pub.dart.snapshot');
   var dartArgs = [pubPath, '--verbose'];
   dartArgs.addAll(args);
   if (tokenEndpoint == null) tokenEndpoint = new Future.value();
   var environmentFuture = tokenEndpoint.then((tokenEndpoint) {
-    var environment = getPubTestEnvironment(tokenEndpoint);
+    var pubEnvironment = getPubTestEnvironment(tokenEndpoint);
     if (_hasServer) {
       return port.then((p) {
-        environment['PUB_HOSTED_URL'] = "http://localhost:$p";
-        return environment;
+        pubEnvironment['PUB_HOSTED_URL'] = "http://localhost:$p";
+        return pubEnvironment;
       });
     }
-    return environment;
+    return pubEnvironment;
+  }).then((pubEnvironment) {
+    if (environment != null) pubEnvironment.addAll(environment);
+    return pubEnvironment;
   });
   return new PubProcess.start(
       dartBin,
       dartArgs,
       environment: environmentFuture,
       workingDirectory: _pathInSandbox(appPath),
       description: args.isEmpty ? 'pub' : 'pub ${args.first}');
 }
 class PubProcess extends ScheduledProcess {
   Stream<Pair<log.Level, String>> _log;
@@ skipping 275 matching lines @@
   bool matches(item, Map matchState) {
     if (item is! Pair) return false;
     return _firstMatcher.matches(item.first, matchState) &&
         _lastMatcher.matches(item.last, matchState);
   }
   Description describe(Description description) {
     return description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }
 StreamMatcher emitsLines(String output) => inOrder(output.split("\n"));