Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/lib/src/sdk.dart

Index: sdk/lib/_internal/pub/lib/src/sdk.dart
diff --git a/utils/pub/sdk.dart b/sdk/lib/_internal/pub/lib/src/sdk.dart
similarity index 92%
rename from utils/pub/sdk.dart
rename to sdk/lib/_internal/pub/lib/src/sdk.dart
index b9b50ecca2c7a1c2867fa340f81e734701292897..6b304d9e4cadb0623658d1e414dcf992566496f1 100644
--- a/utils/pub/sdk.dart
+++ b/sdk/lib/_internal/pub/lib/src/sdk.dart
@@ -28,10 +28,8 @@ String get rootDirectory {
     return dir;
   }
 
-  var pubDir = path.dirname(new Options().script);
-  dir = path.normalize(path.join(pubDir, "../../"));
-  log.fine("Located SDK at $dir");
-  return dir;
+  throw new Exception('DART_SDK environment variable not set, unable to find '
+      'the SDK.');
 }
 
 /// Gets the SDK's revision number formatted to be a semantic version.