Replace the existing unittest APIs with the new runner infrastructure.

pubspec.yaml

Index: pubspec.yaml
diff --git a/pubspec.yaml b/pubspec.yaml
index b134cce7fd5dec2c572e149a84e9e78a92156848..2be4eaeb2c858484f55bc5211614e1eaad04abd9 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,13 @@
 name: unittest
-version: 0.12.0-dev
+version: 0.12.0-alpha.0
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/unittest
 environment:
   sdk: '>=1.0.0 <2.0.0'
 dependencies:
-  stack_trace: '>=0.9.0 <2.0.0'
+  args: '>=0.12.1 <0.13.0'
+  stack_trace: '>=1.2.0 <2.0.0'
 
   # Using the pre-release version of matcher. When published we will go back
   # using a tight version constraint to ensure that a constraint on unittest
@@ -15,7 +16,3 @@ dependencies:
     git: https://github.com/dart-lang/matcher.git
 dev_dependencies:
   fake_async: '>=0.1.2 <0.2.0'
-  metatest:
-    git:
-      ref: b6348d7e7f3c5b00a48aa579694457d1abd36b69
-      url: https://github.com/dart-lang/metatest