fixes #259, clone ast before mutation

lib/src/info.dart

Index: lib/src/info.dart
diff --git a/lib/src/info.dart b/lib/src/info.dart
index 8bfc909e753c1d089d4220fff25c6b3aa291046a..5da0f644c3a0694d3faef695fa4326df58da74e4 100644
--- a/lib/src/info.dart
+++ b/lib/src/info.dart
@@ -9,6 +9,7 @@ library dev_compiler.src.info;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/error.dart';
+import 'package:analyzer/src/generated/parser.dart';
 
 import 'package:dev_compiler/src/checker/rules.dart';
 import 'package:dev_compiler/src/utils.dart' as utils;
@@ -53,6 +54,43 @@ class LibraryUnit {
     yield* parts;
     yield library;
   }
+
+  /// Creates a clone of this library's AST.
+  LibraryUnit clone() {
+    return new LibraryUnit(
+        _cloneUnit(library), parts.map(_cloneUnit).toList(growable: false));
+  }
+
+  static CompilationUnit _cloneUnit(CompilationUnit oldNode) {
+    var newNode = oldNode.accept(new _AstCloner());
+    ResolutionCopier.copyResolutionData(oldNode, newNode);
+    return newNode;
+  }
+}
+
+class _AstCloner extends AstCloner {
+  void _cloneProperties(AstNode clone, AstNode node) {
+    if (clone != null) {
+      CoercionInfo.set(clone, CoercionInfo.get(node));
+      DynamicInvoke.set(clone, DynamicInvoke.get(node));
+    }
+  }
+
+  @override
+  AstNode cloneNode(AstNode node) {
+    var clone = super.cloneNode(node);
+    _cloneProperties(clone, node);
+    return clone;
+  }
+
+  @override
+  List cloneNodeList(List list) {
+    var clone = super.cloneNodeList(list);
+    for (int i = 0, len = list.length; i < len; i++) {
+      _cloneProperties(clone[i], list[i]);
+    }
+    return clone;
+  }
 }
 
 // The abstract type of coercions mapping one type to another.