- Properly return null if an unknown package is resolved.

tests/isolate/scenarios/bad_resolve_package/bad_resolve_package_test.dart

 // Copyright (c) 2016, 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.
 
 // PackageRoot=none
 
 import 'dart:io';
 import 'dart:isolate';
 
-import "package:foo/foo.dart";
-import "package:bar/bar.dart";
-
-var CONFIG_URI = "package:bar/package.config";
-
 main([args, port]) async {
   if (port != null) {
-    testCorrectBarPackage(port);
+    testBadResolvePackage(port);
     return;
   }
   var p = new RawReceivePort();
-  Isolate.spawnUri(Platform.script,
-                   [],
-                   p.sendPort,
-                   packageConfig: Uri.parse(CONFIG_URI));
+  Isolate.spawnUri(Platform.script, [], p.sendPort);
   p.handler = (msg) {
     p.close();
     if (msg is! List) {
       print(msg.runtimeType);
       throw "Failure return from spawned isolate:\n\n$msg";
     }
-    if (msg[0] != "Foo") {
+    // Expecting a null resolution for inexistent package mapping.
+    if (msg[0] != null) {
       throw "Bad package config in child isolate: ${msg[0]}\n"
             "Expected: 'Foo'";
     }
-    if (msg[1] != "Bar2") {
-      throw "Package path not matching: ${msg[1]}\n"
-            "Expected: 'Bar2'";
-    }
     print("SUCCESS");
   };
-  if (Bar.value != "Bar1") {
-    throw "Spawning isolate package:bar invalid.";
-  }
-  print("Spawned isolate resolved $CONFIG_URI to: "
-        "${await Isolate.resolvePackageUri(Uri.parse(CONFIG_URI))}");
 }
 
-testCorrectBarPackage(port) async {
+testBadResolvePackage(port) async {
   try {
     var packageRootStr = Platform.packageRoot;
     var packageConfigStr = Platform.packageConfig;
     var packageConfig = await Isolate.packageConfig;
-    var resolvedPkg = await Isolate.resolvePackageUri(Uri.parse(CONFIG_URI));
+    var badPackageUri = Uri.parse("package:asdf/qwerty.dart");
+    var resolvedPkg = await Isolate.resolvePackageUri(badPackageUri);
     print("Spawned isolate's package root flag: $packageRootStr");
     print("Spawned isolate's package config flag: $packageConfigStr");
     print("Spawned isolate's loaded package config: $packageConfig");
     print("Spawned isolate's resolved package path: $resolvedPkg");
-    port.send([Foo.value, Bar.value]);
+    port.send([resolvedPkg]);
   } catch (e, s) {
     port.send("$e\n$s\n");
   }
 }