Removes dartc reliance on its own libraries, now can be targeted at any implementation's libraries

compiler/lib/error.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// Exceptions thrown by the VM -- user code should never throw these directly.
-// TODO(jat): these should be compatible with the definitions in runtime/lib/error.dart
-
-class AssertionError {
-  const AssertionError();
-
-  String toString() {
-    return "Failed assertion";
-  }
-}
-
-class TypeError extends AssertionError {
-  final String dstType;
-  final String srcType;
-
-  const TypeError(this.srcType, this.dstType);
-
-  String toString() {
-    return "Failed type check: type $srcType is not assignable to type $dstType";
-  }
-}
-
-class FallThroughError {
-  const FallThroughError();
-
-  String toString() {
-    return "Switch case fall-through";
-  }
-}
-