Throw FallThroughError when we fall through a case in a switch statement. Fixes

lib/corelib.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.
 
 #library("dart:core");
 #import("dart:coreimpl");
 
 #native("core.js");
 
 // TODO(jimhug): Better way to map in standard corelib
@@ skipping 74 matching lines @@
   String toString() {
     return "Failed type check: type $srcType is not assignable to type " +
         "$dstType of $dstName in $url at line " +
         "$line, column $column.";
   }
   final String srcType;
   final String dstType;
   final String dstName;
 }
 class FallThroughError {
-  const FallThroughError() : super(); 
+  final String url;
+  final int line;
+ 
+  const FallThroughError(this.url, this.line);
+ 
+  String toString() {
+    return "Switch case fall-through in $url at line $line.";
+  }
 }
 
 // Dart core library.
 
 class Object native "Object" {
 
   const Object() native;
 
   bool operator ==(Object other) native;
   String toString() native;
 
   // TODO(jmesserly): optimize this. No need to call it.
   get dynamic() => this;
 }