Clean-up of getCompilationUnit.

lib/compiler/implementation/dart_backend/emitter.dart

Index: lib/compiler/implementation/dart_backend/emitter.dart
diff --git a/lib/compiler/implementation/dart_backend/emitter.dart b/lib/compiler/implementation/dart_backend/emitter.dart
index 19197d1a688ada5fef2d1330a85d3a1d77011895..dfb85c322917d49b11b5efe64dc77ffc10d02d9d 100644
--- a/lib/compiler/implementation/dart_backend/emitter.dart
+++ b/lib/compiler/implementation/dart_backend/emitter.dart
@@ -66,7 +66,8 @@ class Emitter {
       // Different VariableElement's may refer to the same VariableListElement.
       // Output this list only once.
       // TODO: only emit used variables.
-      final variableList = (element as VariableElement).variables;
+      VariableElement variableElement = element;
+      final variableList = variableElement.variables;
       if (!processedVariableLists.contains(variableList)) {
         processedVariableLists.add(variableList);
         sb.add(unparser.unparse(variableList.parseNode(compiler)));