Use isValidConst flag to distinguish between const / final expressions.

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

 // Copyright (c) 2016, 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.
 
 /**
  * This library is capable of producing linked summaries from unlinked
  * ones (or prelinked ones).  It functions by building a miniature
  * element model to represent the contents of the summaries, and then
  * scanning the element model to gather linked information and adding
  * it to the summary data structures.
@@ skipping 2272 matching lines @@
         dependencies,
         variableElement.unlinkedVariable.constExpr,
         variableElement.compilationUnit);
     return dependencies;
   }
 
   @override
   void evaluate(bool inCycle) {
     if (inCycle) {
       _inferredType = DynamicTypeImpl.instance;
-    } else if (variableElement.unlinkedVariable.constExpr.isInvalid) {
-      // TODO(paulberry): implement.
+    } else if (!variableElement.unlinkedVariable.constExpr.isValidConst) {
+      // TODO(paulberry): delete this case and fix errors.
       throw new UnimplementedError();
     } else {
       // Perform RPN evaluation of the cycle, using a stack of
       // inferred types.
       List<DartType> stack = <DartType>[];
       int refPtr = 0;
       int intPtr = 0;
       UnlinkedConst unlinkedConst = variableElement.unlinkedVariable.constExpr;
       TypeProvider typeProvider =
           variableElement.compilationUnit.enclosingElement._linker.typeProvider;
@@ skipping 539 matching lines @@
   /**
    * Throw away any information stored in the summary by a previous call to
    * [link].
    */
   void unlink() {
     for (LibraryElementInBuildUnit library in _librariesInBuildUnit) {
       library.unlink();
     }
   }
 }