Add access to the isolate package-root/package-map in Platform.

tests/standalone/io/platform_test.dart

Index: tests/standalone/io/platform_test.dart
diff --git a/tests/standalone/io/platform_test.dart b/tests/standalone/io/platform_test.dart
index f07475061da1081074421882db52acabbd972dcf..65b6226ce9e091f0ea55148ff6972409e9e6eac1 100644
--- a/tests/standalone/io/platform_test.dart
+++ b/tests/standalone/io/platform_test.dart
@@ -48,11 +48,20 @@ test() {
   Expect.isTrue(Platform.script.toFilePath().startsWith(oldDir.path));
   // Restore dir.
   Directory.current = oldDir;
-  Directory packageRoot = new Directory(Platform.packageRoot);
-  Expect.isTrue(packageRoot.existsSync());
-  Expect.isTrue(new Directory("${packageRoot.path}/expect").existsSync());
-  Expect.isTrue(Platform.executableArguments.any(
-      (arg) => arg.contains(Platform.packageRoot)));
+  Uri packageRoot = Platform.packageRoot;
+  Map packageMap = Platform.packageMap;
+  // One of packageRoot or packageMap is non-null.
+  Expect.isTrue((packageRoot == null) != (packageMap == null));
+  if (packageRoot != null) {
+    Directory packageDir = new Directory.fromUri(packageRoot);
+    Expect.isTrue(packageRoot.existsSync());
+    Expect.isTrue(new Directory("${packageRoot.path}/expect").existsSync());
+  } else {
+    Uri expectUri = packageMap["expect"];
+    Expect.isNotNull(expectUri);
+    Directory expectDir = new Directory.fromUri(expectUri);
+    Expect.isTrue(expectDir.existsSync());
+  }
 }
 
 void f(reply) {