Move constant expression resynthesis into ConstVariableElement mixin.

pkg/analyzer/lib/src/summary/resynthesize.dart

Index: pkg/analyzer/lib/src/summary/resynthesize.dart
diff --git a/pkg/analyzer/lib/src/summary/resynthesize.dart b/pkg/analyzer/lib/src/summary/resynthesize.dart
index 77c98634228798b61cace629d2252e1d992693f7..368b9cd95fabfbf763b89b07352dfc0724c6c392 100644
--- a/pkg/analyzer/lib/src/summary/resynthesize.dart
+++ b/pkg/analyzer/lib/src/summary/resynthesize.dart
@@ -2172,12 +2172,8 @@ class _UnitResynthesizer {
       TopLevelVariableElementImpl element;
       if (unlinkedVariable.initializer?.bodyExpr != null &&
           unlinkedVariable.isConst) {
-        ConstTopLevelVariableElementImpl constElement =
-            new ConstTopLevelVariableElementImpl.forSerialized(
-                unlinkedVariable, unit);
-        element = constElement;
-        constElement.constantInitializer =
-            _buildConstExpression(null, unlinkedVariable.initializer.bodyExpr);
+        element = new ConstTopLevelVariableElementImpl.forSerialized(
+            unlinkedVariable, unit);
       } else {
         element = new TopLevelVariableElementImpl.forSerialized(
             unlinkedVariable, unit);