Make RegExp's constructor non-const.

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 e00e4e06698c32fe0df42c87b992ec1f130ed6a4..2cba5ca1a36b389cd636748355d0add579641ddf 100644
--- a/utils/pub/git_source.dart
+++ b/utils/pub/git_source.dart
@@ -152,7 +152,7 @@ class GitSource extends Source {
     // have the right working directory when pub spawns git, so the relative
     // 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]+\:');
+    var URL_LIKE = new RegExp(r'^[a-z]+\:');
     if (!URL_LIKE.hasMatch(from)) {
       from = getFullPath(from);
     }