Implement the compiler API and use it in frog_leg.dart.

dart/frog/leg/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;
   Map<int, BailoutInfo> bailouts = null;
   bool allowSpeculativeOptimization = true;
@@ skipping 349 matching lines @@
   }
 
   Object compileVariable(VariableElement element) {
     return withCurrentElement(element, () {
         compile(new WorkItem.toCompile(element));
         return constantHandler.compileVariable(element);
       });
   }
 
   reportWarning(Node node, var message) {}
-  reportError(Node node, var message) {}
+
+  reportError(Node node, var message) => cancel(message, node: node);
 
   Script readScript(Uri uri, [ScriptTag node]) {
     unimplemented('Compiler.readScript');
   }
 
   String get legDirectory() {
     unimplemented('Compiler.legDirectory');
   }
 
   Element findHelper(SourceString name) => jsHelperLibrary.find(name);
@@ skipping 41 matching lines @@
 
 // TODO(ahe): Remove when the VM supports implicit interfaces.
 class LTracer implements Tracer {
   const LTracer();
   final bool enabled = false;
   void traceGraph(String name, var graph) {
   }
   void close() {
   }
 }