Mock up all remaining core library implementation classes.

dart/frog/leg/lib/mock.dart

Index: dart/frog/leg/lib/mock.dart
diff --git a/dart/frog/leg/lib/mock.dart b/dart/frog/leg/lib/mock.dart
index 518f98495e4fe0e2be139f1dc5fc814903a41683..fe97f98a5517cf700cd307d98a4e5e7d4f7abd67 100644
--- a/dart/frog/leg/lib/mock.dart
+++ b/dart/frog/leg/lib/mock.dart
@@ -11,7 +11,11 @@ class TypeError extends AssertionError {}
 class FallThroughError {}
 
 // TODO(ahe): VM specfic exception?
-class InternalError {}
+class InternalError {
+  const InternalError(this._msg);
+  String toString() => "InternalError: '${_msg}'";
+  final String _msg;
+}
 
 // TODO(ahe): VM specfic exception?
 class StaticResolutionException implements Exception {}