Handle URLs in 'git clone' argument.

utils/pub/git_source.dart

Index: utils/pub/git_source.dart
diff --git a/utils/pub/git_source.dart b/utils/pub/git_source.dart
index 5d7e2d72e15fbd8a13793208f0a7403fe7b6c82c..e00e4e06698c32fe0df42c87b992ec1f130ed6a4 100644
--- a/utils/pub/git_source.dart
+++ b/utils/pub/git_source.dart
@@ -150,9 +150,12 @@ class GitSource extends Source {
   Future _clone(String from, String to, {bool mirror: false}) {
     // TODO(rnystrom): For some mysterious reason, the Windows buildbots do not
     // have the right working directory when pub spawns git, so the relative
-    // path fails. To work around it, just always pass in a full path. Should
-    // figure out what's going on here.
-    from = getFullPath(from);
+    // path fails. To work around it, if [from] looks like a relative path then
+    // manually make it absolute here. Should figure out what's really going on.
+    const URL_LIKE = const RegExp(r'^[a-z]+\:');
+    if (!URL_LIKE.hasMatch(from)) {
+      from = getFullPath(from);
+    }
 
     // Git on Windows does not seem to automatically create the destination
     // directory.