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

sdk/lib/io/platform.dart

Index: sdk/lib/io/platform.dart
diff --git a/sdk/lib/io/platform.dart b/sdk/lib/io/platform.dart
index 56ddea0193b0df98b43e703e0c98828f4eb1e696..cbdb016c0fd482edaa6be3adc7f84ba99c77bc19 100644
--- a/sdk/lib/io/platform.dart
+++ b/sdk/lib/io/platform.dart
@@ -176,13 +176,27 @@ class Platform {
   static List<String> get executableArguments => _Platform.executableArguments;
 
   /**
-   * Returns the value of the --package-root flag passed to the executable
-   * used to run the script in this isolate.  This is the directory in which
-   * Dart packages are looked up.
+   * Returns the package root of the current isolate, if any.
    *
-   * If there is no --package-root flag, then the empty string is returned.
+   * If the isolate is using a [packageMap], this getter returns `null`,
+   * otherwise it returns the package root - a directory that package
+   * URIs are resolved against.
    */
-  static String get packageRoot => _Platform.packageRoot;
+  static Uri get packageRoot => _Platform.packageRootUri;
+
+  /**
+   * Returns the package mapping of the current isolate, if any.
+   *
+   * If the current isolate is using [packageRoot], this getter returns `null`.
+   *
+   * The package map maps package names to a directory that package URIs for
+   * that package are resolved against, or to `null`, if the package name is
+   * not recognized.
+   *
+   * Returns an empty map if the isolate does not have a way to resolve package
+   * URIs.
+   */
+  static Map<String, Uri> get packageMap => _Platform.packageMap;
 
   /**
    * Returns the version of the current Dart runtime.