Store constant variable initializers in elements.

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

Index: pkg/compiler/lib/src/elements/elements.dart
diff --git a/pkg/compiler/lib/src/elements/elements.dart b/pkg/compiler/lib/src/elements/elements.dart
index 54ecf63a80f1e3ce3eb8a71d2d874d210fb67f8c..a6683cf1b35018a35338e8b3227bc3602de00ea1 100644
--- a/pkg/compiler/lib/src/elements/elements.dart
+++ b/pkg/compiler/lib/src/elements/elements.dart
@@ -996,8 +996,11 @@ abstract class VariableElement extends ExecutableElement {
 
   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;
 }