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

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

Index: sdk/lib/_internal/pub_generated/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub_generated/test/test_pub.dart b/sdk/lib/_internal/pub_generated/test/test_pub.dart
index fe4b2b6b308233bffa2d873f1807f02e3dcd7cc2..fa699e48dc7751bc8ed1d7c06110a543ac62573e 100644
--- a/sdk/lib/_internal/pub_generated/test/test_pub.dart
+++ b/sdk/lib/_internal/pub_generated/test/test_pub.dart
@@ -242,9 +242,9 @@ void scheduleSymlink(String target, String symlink) {
       'symlinking $target to $symlink');
 }
 void schedulePub({List args, output, error, outputJson, int exitCode:
-    exit_codes.SUCCESS}) {
+    exit_codes.SUCCESS, Map<String, String> environment}) {
   assert(output == null || outputJson == null);
-  var pub = startPub(args: args);
+  var pub = startPub(args: args, environment: environment);
   pub.shouldExit(exitCode);
   var failures = [];
   var stderr;
@@ -293,7 +293,8 @@ Map getPubTestEnvironment([String tokenEndpoint]) {
   }
   return environment;
 }
-ScheduledProcess startPub({List args, Future<String> tokenEndpoint}) {
+ScheduledProcess startPub({List args, Future<String> tokenEndpoint, Map<String,
+    String> environment}) {
   ensureDir(_pathInSandbox(appPath));
   var dartBin = Platform.executable;
   if (dartBin.contains(Platform.pathSeparator)) {
@@ -304,14 +305,17 @@ ScheduledProcess startPub({List args, Future<String> tokenEndpoint}) {
   dartArgs.addAll(args);
   if (tokenEndpoint == null) tokenEndpoint = new Future.value();
   var environmentFuture = tokenEndpoint.then((tokenEndpoint) {
-    var environment = getPubTestEnvironment(tokenEndpoint);
+    var pubEnvironment = getPubTestEnvironment(tokenEndpoint);
     if (_hasServer) {
       return port.then((p) {
-        environment['PUB_HOSTED_URL'] = "http://localhost:$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,