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

sdk/lib/_internal/pub/test/test_pub.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.
 
 /// Test infrastructure for testing pub.
 ///
 /// Unlike typical unit tests, most pub tests are integration tests that stage
 /// some stuff on the file system, run pub, and then validate the results. This
 /// library provides an API to build tests like that.
 library test_pub;
@@ skipping 393 matching lines @@
 /// Schedules a call to the Pub command-line utility.
 ///
 /// Runs Pub with [args] and validates that its results match [output] (or
 /// [outputJson]), [error], and [exitCode].
 ///
 /// [output] and [error] can be [String]s, [RegExp]s, or [Matcher]s.
 ///
 /// If [outputJson] is given, validates that pub outputs stringified JSON
 /// matching that object, which can be a literal JSON object or any other
 /// [Matcher].
+///
+/// If [environment] is given, any keys in it will override the environment
+/// variables passed to the spawned process.
 void schedulePub({List args, output, error, outputJson,
-    int exitCode: exit_codes.SUCCESS}) {
+    int exitCode: exit_codes.SUCCESS, Map<String, String> environment}) {
   // Cannot pass both output and outputJson.
   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");
@@ skipping 65 matching lines @@
     environment['_PUB_TEST_TOKEN_ENDPOINT'] = tokenEndpoint.toString();
   }
 
   return environment;
 }
 
 /// Starts a Pub process and returns a [ScheduledProcess] that supports
 /// interaction with that process.
 ///
 /// Any futures in [args] will be resolved before the process is started.
-ScheduledProcess startPub({List args, Future<String> tokenEndpoint}) {
+///
+/// If [environment] is given, any keys in it will override the environment
+/// variables passed to the spawned process.
+ScheduledProcess startPub({List args, Future<String> tokenEndpoint,
+    Map<String, String> environment}) {
   ensureDir(_pathInSandbox(appPath));
 
   // Find a Dart executable we can use to spawn. Use the same one that was
   // used to run this script itself.
   var dartBin = Platform.executable;
 
   // If the executable looks like a path, get its full path. That way we
   // can still find it when we spawn it with a different working directory.
   if (dartBin.contains(Platform.pathSeparator)) {
     dartBin = p.absolute(dartBin);
   }
 
   // Always run pub from a snapshot. Since we require the SDK to be built, the
   // snapshot should be there. Note that this *does* mean that the snapshot has
   // to be manually updated when changing code before running the tests.
   // Otherwise, you will test against stale data.
   //
   // Using the snapshot makes running the tests much faster, which is why we
   // make this trade-off.
   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 there is a server running, tell pub what its URL is so hosted
     // dependencies will look there.
     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}');
 }
 
 /// A subclass of [ScheduledProcess] that parses pub's verbose logging output
 /// and makes [stdout] and [stderr] work as though pub weren't running in
 /// verbose mode.
@@ skipping 414 matching lines @@
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     return description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }
 
 /// A [StreamMatcher] that matches multiple lines of output.
 StreamMatcher emitsLines(String output) => inOrder(output.split("\n"));