Use environment variables to find the Dart SDK and the Pub cache directory.

utils/pub/sdk_source.dart

Index: utils/pub/sdk_source.dart
diff --git a/utils/pub/sdk_source.dart b/utils/pub/sdk_source.dart
index 3662f853fa4530280307cbfe9e5b2369fb43aa6c..54fa218c231f7ca6351467e33c61b12c906635eb 100644
--- a/utils/pub/sdk_source.dart
+++ b/utils/pub/sdk_source.dart
@@ -22,9 +22,15 @@ class SdkSource extends Source {
   /**
    * The root directory of the Dart SDK.
    */
-  final String rootDir;
+  final String _rootDir;
 
-  SdkSource(this.rootDir);
+  String get rootDir() {
+    if (_rootDir != null) return _rootDir;
+    throw "Pub can't find the Dart SDK. Please set the DART_SDK environment "

[Bob Nystrom, 2012/07/12 16:49:27]

Instead of checking here, why not having the constructor validate it? Do we want
to allow SdkSource instances without a root directory?

[nweiz, 2012/07/12 18:44:30]

We want pub to work without DART_SDK if it doesn't need anything from the SDK
anyway (e.g. just installing from Git or the repo).

[Bob Nystrom, 2012/07/12 21:07:57]

OK, cool.

+      "variable to the Dart SDK directory.";
+  }
+
+  SdkSource(this._rootDir);
 
   /**
    * An SDK package has no dependencies. Its version number is inferred from the