fixes #259, clone ast before mutation

lib/src/checker/checker.dart

Index: lib/src/checker/checker.dart
diff --git a/lib/src/checker/checker.dart b/lib/src/checker/checker.dart
index 46b909ab86f19e5f0fa82433114c257ee6836a63..61ec63a35ac60fbd276848f1ecce3f606e991d04 100644
--- a/lib/src/checker/checker.dart
+++ b/lib/src/checker/checker.dart
@@ -1001,7 +1001,11 @@ class CodeChecker extends RecursiveAstVisitor {
     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);
     }
   }