Add package root support to analysis server.

pkg/analyzer/lib/src/generated/source_io.dart

Index: pkg/analyzer/lib/src/generated/source_io.dart
diff --git a/pkg/analyzer/lib/src/generated/source_io.dart b/pkg/analyzer/lib/src/generated/source_io.dart
index a5b4347d78c0bac7bca553dceba28e2b289a6c0c..08c3b6ae9d225ac970234a3313d067543025df3c 100644
--- a/pkg/analyzer/lib/src/generated/source_io.dart
+++ b/pkg/analyzer/lib/src/generated/source_io.dart
@@ -345,6 +345,18 @@ class PackageUriResolver extends UriResolver {
     }
   }
 
+  /**
+   * If the list of package directories contains one element, return it.
+   * Otherwise raise an exception.  Intended for testing.
+   */
+  String get packagesDirectory_forTesting {
+    int length = _packagesDirectories.length;
+    if (length != 1) {
+      throw new Exception('Expected 1 package directory, found $length');
+    }
+    return _packagesDirectories[0].getPath();
+  }
+
   @override
   Source resolveAbsolute(Uri uri) {
     if (!isPackageUri(uri)) {