update collection/meta deps to the published versions (#126)

diff --git a/pubspec.yaml b/pubspec.yaml
index bfdf20e..b8b410e 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -8,7 +8,7 @@
   sdk: '>=2.9.0-20.0.dev <2.9.0'
 
 dependencies:
-  collection: ^1.5.0
+  collection: '>=1.15.0-nullsafety <1.15.0'
 
 dev_dependencies:
   fake_async: ^1.0.0
@@ -29,10 +29,6 @@
     git:
       url: git://github.com/dart-lang/clock.git
       ref: null_safety
-  collection:
-    git:
-      url: git://github.com/dart-lang/collection.git
-      ref: null_safety
   fake_async:
     git:
       url: git://github.com/dart-lang/fake_async.git
@@ -41,11 +37,7 @@
     git:
       url: git://github.com/dart-lang/matcher.git
       ref: null_safety
-  meta:
-    git:
-      url: git://github.com/dart-lang/sdk.git
-      ref: null_safety-pkgs
-      path: pkg/meta
+  meta: '>=1.3.0-nullsafety <1.3.0'
   path:
     git:
       url: git://github.com/dart-lang/path.git