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..53a4664b846b7f775ef5804a8a4692c8f384126d 100644
--- a/utils/pub/sdk_source.dart
+++ b/utils/pub/sdk_source.dart
@@ -13,8 +13,6 @@
  *
  * This currently uses the "sdkdir" command-line argument to find the SDK.
  */
-// TODO(nweiz): This should read the SDK directory from an environment variable
-// once we can set those for tests.
 class SdkSource extends Source {
   final String name = "sdk";
   final bool shouldCache = false;
@@ -22,9 +20,15 @@ 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 "
+      "variable to the Dart SDK directory.";
+  }
+
+  SdkSource(this._rootDir);
 
   /**
    * An SDK package has no dependencies. Its version number is inferred from the