fixes #259, clone ast before mutation

lib/src/checker/checker.dart

 // Copyright (c) 2015, 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 dev_compiler.src.checker.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/scanner.dart' show Token, TokenType;
@@ skipping 983 matching lines @@
     DynamicInvoke.set(target, true);
   }
 
   void _recordMessage(StaticInfo info) {
     if (info == null) return;
     var error = info.toAnalysisError();
     if (error.errorCode.errorSeverity == ErrorSeverity.ERROR) _failure = true;
     reporter.onError(error);
 
     if (info is CoercionInfo) {
-      assert(CoercionInfo.get(info.node) == null);
+      // TODO(jmesserly): if we're run again on the same AST, we'll produce the
+      // same annotations. This should be harmless. This might go away once
+      // CodeChecker is integrated better with analyzer, as it will know that
+      // checking has already been performed.
+      // assert(CoercionInfo.get(info.node) == null);
       CoercionInfo.set(info.node, info);
     }
   }
 }