Decouple the constant handler from the compiler so we can have more than one.

sdk/lib/_internal/compiler/implementation/resolution/members.dart

 // Copyright (c) 2012, 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 resolution;
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(Node node);
@@ skipping 626 matching lines @@
 
   void resolveMetadataAnnotation(PartialMetadataAnnotation annotation) {
     compiler.withCurrentElement(annotation.annotatedElement, () => measure(() {
       assert(annotation.resolutionState == STATE_NOT_STARTED);
       annotation.resolutionState = STATE_STARTED;
 
       Node node = annotation.parseNode(compiler);
       ResolverVisitor visitor =
           visitorFor(annotation.annotatedElement.enclosingElement);
       node.accept(visitor);
-      annotation.value = compiler.constantHandler.compileNodeWithDefinitions(
+      annotation.value = compiler.metadataHandler.compileNodeWithDefinitions(
           node, visitor.mapping, isConst: true);
 
       annotation.resolutionState = STATE_DONE;
     }));
   }
 
   error(Node node, MessageKind kind, [arguments = const []]) {
     ResolutionError message = new ResolutionError(kind, arguments);
     compiler.reportError(node, message);
   }
@@ skipping 2513 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }