Support top-level field declaration and assignment in analyzer2dart.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library tree_ir_nodes;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart' as values;
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
 import '../dart_types.dart' show DartType, GenericType;
@@ skipping 549 matching lines @@
   applyPass(Pass pass);
 }
 
 class FieldDefinition extends Node implements ExecutableDefinition {
   final FieldElement element;
   // The `body` of a field is its initializer.
   Statement body;
 
   FieldDefinition(this.element, this.body);
   applyPass(Pass pass) => pass.rewriteFieldDefinition(this);
+
+  /// `true` if this field has no initializer.
+  ///
+  /// If `true` [body] is `null`.
+  ///
+  /// This is different from a initializer that is `null`. Consider this class:
+  ///
+  ///     class Class {
+  ///       final field;
+  ///       Class.a(this.field);
+  ///       Class.b() : this.field = null;
+  ///       Class.c();
+  ///     }
+  ///
+  /// If `field` had an initializer, possibly `null`, constructors `Class.a` and
+  /// `Class.b` would be invalid, and since `field` has no initializer
+  /// constructor `Class.c` is invalid. We therefore need to distinguish the two
+  /// cases.
+  bool get hasInitializer => body != null;
 }
 
 class FunctionDefinition extends Node implements ExecutableDefinition {
   final FunctionElement element;
   final List<Variable> parameters;
   Statement body;
   final List<ConstDeclaration> localConstants;
   final List<ConstantExpression> defaultParameterValues;
 
   FunctionDefinition(this.element, this.parameters, this.body,
@@ skipping 192 matching lines @@
   visitFunctionDeclaration(FunctionDeclaration node) {
     visitFunctionDefinition(node.definition);
     visitStatement(node.next);
   }
 
   visitExpressionStatement(ExpressionStatement node) {
     visitExpression(node.expression);
     visitStatement(node.next);
   }
 }