Add a Pub source that checks packages out from Git.

utils/tests/pub/pub_test.dart

Index: utils/tests/pub/pub_test.dart
diff --git a/utils/tests/pub/pub_test.dart b/utils/tests/pub/pub_test.dart
index a8b242b032a298357f6fd3254bb7b28908480abc..76f79ddfe2949a65f87a87c37cc30ac04b104eb3 100644
--- a/utils/tests/pub/pub_test.dart
+++ b/utils/tests/pub/pub_test.dart
@@ -92,7 +92,7 @@ installCommand() {
       ])
     ],
     app: dir('myapp', [
-      file('pubspec', 'dependencies:\n- foo')
+      file('pubspec', 'dependencies:\n  foo:')
     ]),
     args: ['install'],
     expectedPackageDir: [
@@ -109,7 +109,7 @@ installCommand() {
       dir('lib', [
         dir('foo', [
           file('foo.dart', 'main() => "foo";'),
-          file('pubspec', 'dependencies:\n- bar')
+          file('pubspec', 'dependencies:\n  bar:')
         ]),
         dir('bar', [
           file('bar.dart', 'main() => "bar";'),
@@ -117,7 +117,7 @@ installCommand() {
       ])
     ],
     app: dir('myapp', [
-      file('pubspec', 'dependencies:\n- foo')
+      file('pubspec', 'dependencies:\n  foo:')
     ]),
     args: ['install'],
     expectedPackageDir: [