Make the HTTP MockClient work with GET requests.

pkg/http/test/mock_client_test.dart

Index: pkg/http/test/mock_client_test.dart
diff --git a/pkg/http/test/mock_client_test.dart b/pkg/http/test/mock_client_test.dart
index 2425467c98a34a51507969209a9eb07bc1c88cee..7bc39566ff7e780907c0ae8ffb507d8ed68e956d 100644
--- a/pkg/http/test/mock_client_test.dart
+++ b/pkg/http/test/mock_client_test.dart
@@ -54,4 +54,13 @@ void main() {
         .transform((response) => response.body);
     expect(future, completion(equals('Request body was "hello, world"')));
   });
+
+  test('handles a request with no body', () {
+    var client = new MockClient((request) {
+      return new Future.immediate(new http.Response('you did it', 200));
+    });
+
+    expect(client.read("http://example.com/foo"),
+        completion(equals('you did it')));
+  });
 }