Use ConstExp for storing constants.

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

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/sdk/lib/_internal/compiler/implementation/compiler.dart b/sdk/lib/_internal/compiler/implementation/compiler.dart
index c278a58c92f8a91eedb69cb29c6abbe5362c9f85..562c4bbd20075c7554686dbca0de5c96f48317b3 100644
--- a/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -1200,14 +1200,15 @@ abstract class Compiler implements DiagnosticListener {
       functionApplyMethod = functionClass.lookupLocalMember('apply');
 
       proxyConstant =
-          resolver.constantCompiler.compileConstant(coreLibrary.find('proxy'));
+          resolver.constantCompiler.compileConstant(
+              coreLibrary.find('proxy')).value;
 
       // TODO(johnniwinther): Move this to the JavaScript backend.
       LibraryElement jsHelperLibrary =
           loadedLibraries[js_backend.JavaScriptBackend.DART_JS_HELPER];
       if (jsHelperLibrary != null) {
         patchConstant = resolver.constantCompiler.compileConstant(
-            jsHelperLibrary.find('patch'));
+            jsHelperLibrary.find('patch')).value;
       }
 
       if (preserveComments) {
@@ -1271,16 +1272,12 @@ abstract class Compiler implements DiagnosticListener {
     mapClass = lookupCoreClass('Map');
     nullClass = lookupCoreClass('Null');
     stackTraceClass = lookupCoreClass('StackTrace');
+    symbolClass = lookupCoreClass('Symbol');
     if (!missingCoreClasses.isEmpty) {
       internalError(coreLibrary,
           'dart:core library does not contain required classes: '
           '$missingCoreClasses');
     }
-
-    // The Symbol class may not exist during unit testing.
-    // TODO(ahe): It is possible that we have to require the presence
-    // of Symbol as we change how we implement noSuchMethod.
-    symbolClass = lookupCoreClass('Symbol');
   }
 
   Element _unnamedListConstructor;