Rework error handling in the service protocol and in Observatory.

runtime/observatory/lib/src/elements/library_view.dart

Index: runtime/observatory/lib/src/elements/library_view.dart
diff --git a/runtime/observatory/lib/src/elements/library_view.dart b/runtime/observatory/lib/src/elements/library_view.dart
index 4a83c16b93eb4c5399674422c3e27b217bb2e932..197a523aceff13b230d55c35dadd501072e6e9a6 100644
--- a/runtime/observatory/lib/src/elements/library_view.dart
+++ b/runtime/observatory/lib/src/elements/library_view.dart
@@ -20,11 +20,11 @@ class LibraryViewElement extends ObservatoryElement {
     return library.isolate.eval(library, expression);
   }
 
-  void refresh(var done) {
-    library.reload().whenComplete(done);
+  Future refresh() {
+    return library.reload();
   }
 
-  void refreshCoverage(var done) {
-    library.refreshCoverage().whenComplete(done);
+  Future refreshCoverage() {
+    return library.refreshCoverage();
   }
 }