Support checked mode for field setters.

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.
 
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
 const bool REPORT_EXCESS_RESOLUTION = false;
@@ skipping 43 matching lines @@
   abstract void enqueueHelpers(Enqueuer world);
   abstract void codegen(WorkItem work);
   abstract void processNativeClasses(Enqueuer world,
                                      Collection<LibraryElement> libraries);
   abstract void assembleProgram();
   abstract List<CompilerTask> get tasks;
 
   ItemCompilationContext createItemCompilationContext() {
     return new ItemCompilationContext();
   }
+
+  SourceString getCheckedModeHelper(Type type) => null;
 }
 
 class Compiler implements DiagnosticListener {
   final Map<String, LibraryElement> libraries;
   int nextFreeClassId = 0;
   World world;
   String assembledCode;
   Namer namer;
   Types types;
 
@@ skipping 822 matching lines @@
     final endOffset = end.charOffset + end.slowCharCount;
 
     // [begin] and [end] might be the same for the same empty token. This
     // happens for instance when scanning '$$'.
     assert(endOffset >= beginOffset);
     return f(beginOffset, endOffset);
   }
 
   String toString() => 'SourceSpan($uri, $begin, $end)';
 }