Use computeConstantValue instead of constantValue

pkg/dev_compiler/lib/src/compiler/side_effect_analysis.dart

Index: pkg/dev_compiler/lib/src/compiler/side_effect_analysis.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/side_effect_analysis.dart b/pkg/dev_compiler/lib/src/compiler/side_effect_analysis.dart
index 499cb862c7e9d1ea80284eff5d16631cf26851ba..c273228dfe937ad42622abbe8e3e5f6a62cf3570 100644
--- a/pkg/dev_compiler/lib/src/compiler/side_effect_analysis.dart
+++ b/pkg/dev_compiler/lib/src/compiler/side_effect_analysis.dart
@@ -124,7 +124,7 @@ class ConstFieldVisitor {
   DartObject computeConstant(VariableDeclaration field) {
     // If the constant is already computed by ConstantEvaluator, just return it.
     VariableElement element = field.element;
-    var result = element.constantValue;
+    var result = element.computeConstantValue();
     if (result != null) return result;
 
     // ConstantEvaluator will not compute constants for non-const fields,