Support top-level field declaration and assignment in analyzer2dart.

pkg/compiler/lib/src/tree_ir/optimization/optimization.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart b/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
index 63652c45e3f0e1810d6a0b9a9148811b999706c5..737ac3fdab68a7bbeb5be0ea25c292297a0a004c 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
@@ -22,6 +22,7 @@ abstract class PassMixin implements Pass {
   void rewrite(ExecutableDefinition root) => root.applyPass(this);
   void rewriteExecutableDefinition(ExecutableDefinition root);
   void rewriteFieldDefinition(FieldDefinition root) {
+    if (!root.hasInitializer) return;
     rewriteExecutableDefinition(root);
   }
   void rewriteFunctionDefinition(FunctionDefinition root) {