Compute Compiler.proxyConstant on demand.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index db180aa93f496307e18dd23bfc5d99c00e88b4f9..249176adc4563fda5ae09fd1640c3a16871d5a01 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -848,11 +848,6 @@ abstract class Compiler implements DiagnosticListener {
       functionClass.ensureResolved(this);
       functionApplyMethod = functionClass.lookupLocalMember('apply');
 
-      proxyConstant =
-          constants.getConstantValue(
-              resolver.constantCompiler.compileConstant(
-                  coreLibrary.find('proxy')));
-
       if (preserveComments) {
         return libraryLoader.loadLibrary(Uris.dart_mirrors)
             .then((LibraryElement libraryElement) {
@@ -862,6 +857,18 @@ abstract class Compiler implements DiagnosticListener {
     }).then((_) => backend.onLibrariesLoaded(loadedLibraries));
   }
 
+  bool isProxyConstant(ConstantValue value) {
+    FieldElement field = coreLibrary.find('proxy');
+    if (field == null) return false;
+    if (!enqueuer.resolution.hasBeenResolved(field)) return false;
+    if (proxyConstant == null) {
+      proxyConstant =
+          constants.getConstantValue(
+              resolver.constantCompiler.compileConstant(field));
+    }
+    return proxyConstant == value;
+  }
+
   Element findRequiredElement(LibraryElement library, String name) {
     var element = library.find(name);
     if (element == null) {