Update references to Mojo core and bindings

Build fix after recent Dart bindings refactoring.  Now that these libraries are
not included in the snapshot, we need to load them from the mojo package.

R=zra@google.com
TBR=zra@chromium.org

Review URL: https://codereview.chromium.org/1029683002

examples/terminal/terminal_file_impl.dart

@@ -4,7 +4,7 @@
 
 import 'dart:async';
 import 'dart:core';
-import 'dart:mojo.core';
+import 'package:mojo/public/dart/core.dart' as core;
 import 'package:mojo/services/files/public/interfaces/file.mojom.dart' as files;
 import 'package:mojo/services/files/public/interfaces/types.mojom.dart' as files;