Run closed_world2_test using the normal compiler pipeline.

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index 5c53c74b4e6ab09477e6b832b192bc8f0085ff95..ca306b8c9cd5396519e485d557423f12b0c79226 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -209,7 +209,8 @@ class MockCompiler extends Compiler {
 
   CollectingTreeElements resolveStatement(String text) {
     parsedTree = parseStatement(text);
-    return resolveNodeStatement(parsedTree, new MockElement(mainApp));
+    LibraryElement library = mainApp;
+    return resolveNodeStatement(parsedTree, new MockElement(library));
   }
 
   TreeElementMapping resolveNodeStatement(
@@ -231,7 +232,8 @@ class MockCompiler extends Compiler {
   }
 
   resolverVisitor() {
-    Element mockElement = new MockElement(mainApp.entryCompilationUnit);
+    LibraryElement library = mainApp;
+    Element mockElement = new MockElement(library.entryCompilationUnit);
     ResolverVisitor visitor = new ResolverVisitor(
         this.resolution,
         mockElement,