add back coverage override to fix travis (dart-lang/source_maps#51)

pkgs/source_maps/pubspec.yaml

diff --git a/pkgs/source_maps/pubspec.yaml b/pkgs/source_maps/pubspec.yaml
index 4bdc225..8fc3fb3 100644
--- a/pkgs/source_maps/pubspec.yaml
+++ b/pkgs/source_maps/pubspec.yaml
@@ -16,6 +16,9 @@
   term_glyph: ^1.0.0
 
 dependency_overrides:
+  # Overrides required for a version solve	
+  coverage: 0.14.0
+  # Null Safety Overrides
   async:
     git: git://github.com/dart-lang/async.git
   boolean_selector: