Fix many warnings/errors when building Observatory

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

Index: runtime/observatory/tests/observatory_ui/mocks/objects/vm.dart
diff --git a/runtime/observatory/tests/observatory_ui/mocks/objects/vm.dart b/runtime/observatory/tests/observatory_ui/mocks/objects/vm.dart
index 0d604730a54c8e7dd21fcb6c7b09ffee74a1c3db..1b9db5591fb76c26205c4fca54c9efb09e93cdab 100644
--- a/runtime/observatory/tests/observatory_ui/mocks/objects/vm.dart
+++ b/runtime/observatory/tests/observatory_ui/mocks/objects/vm.dart
@@ -21,6 +21,13 @@ class VMMock implements M.VM {
   final int maxRSS;
   final DateTime startTime;
   final Iterable<M.IsolateRef> isolates;
+  final int nativeZoneMemoryUsage = 0;
+  final int heapAllocatedMemoryUsage = 0;
+  final int heapAllocationCount = 0;
+
+  Future<dynamic> invokeRpc(String method, Map params) {
+    return null;
+  }
 
   const VMMock(
       {this.name: 'vm-name',