Introduce AST cloning.

tests/compiler/dart2js/unparser_test.dart

Index: tests/compiler/dart2js/unparser_test.dart
diff --git a/tests/compiler/dart2js/unparser_test.dart b/tests/compiler/dart2js/unparser_test.dart
index 5186a136c9d0d4a83ea5357942289b8446eecd7f..5585df8f971f2335c07e7166ef61d621986c175f 100644
--- a/tests/compiler/dart2js/unparser_test.dart
+++ b/tests/compiler/dart2js/unparser_test.dart
@@ -474,6 +474,19 @@ testFieldTypeOutput() {
   testDart2Dart('main(){new A().field;}class B{}class A{B field;}');
 }
 
+class DynoMap implements Map<Element, ElementAst> {

[Roman, 2012/09/07 08:10:56]

I don't understand "DynoMap" name, what does it mean?

[Anton Muhin, 2012/09/07 12:26:07]

Name is horrible, sorry.  I meant that there is no real data, but they are
rather synthesized on the fly.  Any better ideas?  Something like ParsingMap? 
Sounds horrible too.

Another option would be to forget about map altogether and pass a function to
convert an element into element ast, but you usually are so fond of OOP :)

+  final compiler;
+  DynoMap(this.compiler);
+
+  get resolvedElements => compiler.enqueuer.resolution.resolvedElements;
+  ElementAst operator[](Element element) =>
+      new ElementAst(element.parseNode(compiler), resolvedElements[element]);
+}
+
+PlaceholderCollector collectPlaceholders(compiler, element) =>
+  new PlaceholderCollector(compiler, new Set<String>(), new DynoMap(compiler))
+      ..collect(element);
+
 testLocalFunctionPlaceholder() {
   var src = '''
 main() {
@@ -485,10 +498,7 @@ main() {
   compiler.parseScript(src);
   FunctionElement mainElement = compiler.mainApp.find(leg.Compiler.MAIN);
   compiler.processQueue(compiler.enqueuer.resolution, mainElement);
-  PlaceholderCollector collector =
-      new PlaceholderCollector(compiler, new Set<String>());
-  collector.collect(mainElement,
-      compiler.enqueuer.resolution.resolvedElements[mainElement]);
+  PlaceholderCollector collector = collectPlaceholders(compiler, mainElement);
   FunctionExpression mainNode = mainElement.parseNode(compiler);
   FunctionExpression fooNode = mainNode.body.statements.nodes.head.function;
   LocalPlaceholder fooPlaceholder =
@@ -515,9 +525,7 @@ main() {
   ClassElement interfaceElement = compiler.mainApp.find(buildSourceString('I'));
   interfaceElement.ensureResolved(compiler);
   PlaceholderCollector collector =
-      new PlaceholderCollector(compiler, new Set<String>());
-  collector.collect(interfaceElement,
-      compiler.enqueuer.resolution.resolvedElements[interfaceElement]);
+      collectPlaceholders(compiler, interfaceElement);
   ClassNode interfaceNode = interfaceElement.parseNode(compiler);
   Node defaultTypeNode = interfaceNode.defaultClause.typeName;
   ClassElement classElement = compiler.mainApp.find(buildSourceString('C'));