Store constant variable initializers in elements.

pkg/compiler/lib/src/elements/elements.dart

 // Copyright (c) 2012, 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 elements;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution;
 import '../compiler.dart' show Compiler;
 import '../constants/constructors.dart';
@@ skipping 978 matching lines @@
 abstract class LocalElement extends Element
     implements AstElement, TypedElement, Local {}
 
 /// A top level, static or instance field, a formal parameter or local variable.
 abstract class VariableElement extends ExecutableElement {
   @override
   VariableDefinitions get node;
 
   Expression get initializer;
 
-  /// The constant expression defining the value of the variable if `const`,
-  /// `null` otherwise.
+  /// The constant expression defining the (initial) value of the variable.
+  ///
+  /// If the variable is `const` the value is always non-null, possibly an
+  /// [ErroneousConstantExpression], otherwise, the value is null when the
+  /// initializer isn't a constant expression.
   ConstantExpression get constant;
 }
 
 /// An entity that defines a local entity (memory slot) in generated code.
 ///
 /// Parameters, local variables and local functions (can) define local entity
 /// and thus implement [Local] through [LocalElement]. For non-element locals,
 /// like `this` and boxes, specialized [Local] classes are created.
 ///
 /// Type variables can introduce locals in factories and constructors
@@ skipping 783 matching lines @@
   /// by a field.
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }