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

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index 84d01bc0fe4c925bad2bd0db19786c665ac6da1e..a5246c2911c75f6a7b01b075637b1ce5781b9c05 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -333,13 +333,15 @@ Future<List<int>> consumeInputStream(InputStream stream) {
  * piped streams won't be available in the result object.
  */
 Future<PubProcessResult> runProcess(String executable, List<String> args,
-    [workingDir, bool pipeStdout = false, bool pipeStderr = false]) {
+    [workingDir, Map<String, String> environment, bool pipeStdout = false,
+    bool pipeStderr = false]) {
   int exitCode;
 
   final options = new ProcessOptions();
   if (workingDir != null) {
     options.workingDirectory = _getDirectory(workingDir).path;
   }
+  options.environment = environment;
 
   final process = Process.start(executable, args, options);