Start implementing checked mode and tools support for using it.

lib/compiler/implementation/compiler.dart

 // Copyright (c) 2012, 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.
 
 class WorkItem {
   final Element element;
   TreeElements resolutionTree;
   Function run;
   bool allowSpeculativeOptimization = true;
   List<HTypeGuard> guards = const <HTypeGuard>[];
@@ skipping 18 matching lines @@
     return compiler.codegen(this);
   }
 }
 
 class Compiler implements DiagnosticListener {
   Queue<WorkItem> worklist;
   Universe universe;
   String assembledCode;
   Namer namer;
   Types types;
+  bool enableTypeAssertions = false;
 
   final Tracer tracer;
 
   CompilerTask measuredTask;
   Element _currentElement;
   LibraryElement coreLibrary;
   LibraryElement coreImplLibrary;
   LibraryElement isolateLibrary;
   LibraryElement jsHelperLibrary;
   LibraryElement interceptorsLibrary;
@@ skipping 518 matching lines @@
   }
 }
 
 class SourceSpan {
   final Uri uri;
   final int begin;
   final int end;
 
   const SourceSpan(this.uri, this.begin, this.end);
 }