Add ConstantConstructor to ConstantExpression system.

pkg/compiler/lib/src/resolution/send_resolver.dart

 // Copyright (c) 2015, 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.
 
 part of dart2js.semantics_visitor;
 
 enum SendStructureKind {
   GET,
   SET,
   INVOKE,
@@ skipping 730 matching lines @@
     Node node = definitions.definitions.nodes.single;
     ParameterElement element = elements[node];
     if (element == null) {
       throw new SpannableAssertionFailure(
           node, "No parameter structure for $node.");
     }
     if (isRequired) {
       return new RequiredParameterStructure(
           definitions, node, element, index);
     } else {
-      ConstantExpression defaultValue;
-      if (element.initializer != null) {
-        defaultValue = elements.getConstant(element.initializer);
-      }
+      // TODO(johnniwinther): Should we differentiate between implicit (null)
+      // and explicit values? What about optional parameters on redirecting
+      // factories?
       if (isNamed) {
         return new NamedParameterStructure(
-            definitions, node, element, defaultValue);
+            definitions, node, element, element.constant);
       } else {
         return new OptionalParameterStructure(
-            definitions, node, element, defaultValue, index);
+            definitions, node, element, element.constant, index);
       }
     }
   }
 
   void computeVariableStructures(
       VariableDefinitions definitions,
       void callback(Node node, VariableStructure structure)) {
     for (Node node in definitions.definitions) {
        callback(definitions, computeVariableStructure(node));
     }
@@ skipping 14 matching lines @@
       return internalError(node, "Unexpected variable $element.");
     }
     if (element.isConst) {
       ConstantExpression constant = elements.getConstant(element.initializer);
       return new ConstantVariableStructure(kind, node, element, constant);
     } else {
       return new NonConstantVariableStructure(kind, node, element);
     }
   }
 }