Throw Error on unreachable code rather than a String.

tests/compiler/dart2js/mock_libraries.dart

 // Copyright (c) 2014, 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.
 
 // Library for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 92 matching lines @@
 import 'dart:_js_helper';
 import 'dart:_interceptors';
 import 'dart:_isolate_helper';
 import 'dart:async';
 ''';
 
 const Map<String, String> DEFAULT_JS_HELPER_LIBRARY = const <String, String>{
   'assertTest': 'assertTest(a) {}',
   'assertThrow': 'assertThrow(a) {}',
   'assertHelper': 'assertHelper(a) {}',
+  'assertUnreachable': 'assertUnreachable() {}',
   'assertIsSubtype': 'assertIsSubtype(subtype, supertype, message) {}',
   'assertSubtype': 'assertSubtype(object, isField, checks, asField) {}',
   'assertSubtypeOfRuntimeType': 'assertSubtypeOfRuntimeType(object, type) {}',
   'asyncHelper': 'asyncHelper(object, asyncBody, completer) {}',
   'boolConversionCheck': 'boolConversionCheck(x) {}',
   'boolTypeCast': 'boolTypeCast(value) {}',
   'boolTypeCheck': 'boolTypeCheck(value) {}',
   'checkSubtype': 'checkSubtype(object, isField, checks, asField) {}',
   'checkSubtypeOfRuntimeType': 'checkSubtypeOfRuntimeType(o, t) {}',
   'BoundClosure': r'''abstract class BoundClosure extends Closure {
@@ skipping 316 matching lines @@
 
     const LookupMap(this._entries, [this._nestedMaps = const []])
         : _key = null, _value = null;
 
     const LookupMap.pair(this._key, this._value)
         : _entries = const [], _nestedMaps = const [];
     V operator[](K k) => null;
   }''',
   '_version': 'const _version = "0.0.1+1";',
 };