Start implementing checked mode and tools support for using it.

lib/compiler/implementation/lib/mock.dart

Index: lib/compiler/implementation/lib/mock.dart
===================================================================
--- lib/compiler/implementation/lib/mock.dart	(revision 7371)
+++ lib/compiler/implementation/lib/mock.dart	(working copy)
@@ -7,7 +7,11 @@
 // TODO(ahe): Remove this file.
 
 class AssertionError {}
-class TypeError extends AssertionError {}
+class TypeError extends AssertionError {
+  final String msg;
+  const TypeError(String this.msg);
+  String toString() => msg;
+}
 
 class FallThroughError {
   const FallThroughError();