Address observatory analysis issues.

runtime/observatory/tests/observatory_ui/mocks/objects/isolate.dart

Index: runtime/observatory/tests/observatory_ui/mocks/objects/isolate.dart
diff --git a/runtime/observatory/tests/observatory_ui/mocks/objects/isolate.dart b/runtime/observatory/tests/observatory_ui/mocks/objects/isolate.dart
index fa841307284410a8b3a9c00983451af3ca051d3d..310e8fbfa012074ca521d23ab36d3814592967f6 100644
--- a/runtime/observatory/tests/observatory_ui/mocks/objects/isolate.dart
+++ b/runtime/observatory/tests/observatory_ui/mocks/objects/isolate.dart
@@ -53,4 +53,8 @@ class IsolateMock implements M.Isolate {
       this.pauseEvent,
       this.rootLibrary,
       this.entry});
+
+  Future collectAllGarbage() async {
+    throw "Unimplemented";
+  }
 }