Handle @proxy classes in type checker.

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 c49df9d921878e373d5d327bc44e42ba6cc709c6..4dfb8c1050df61d25819e631382a6209c873e870 100644
--- a/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -440,6 +440,9 @@ abstract class Compiler implements DiagnosticListener {
   ClassElement stackTraceClass;
   ClassElement typedDataClass;
 
+  /// The constant for the [proxy] variable defined in dart:core.
+  Constant proxyConstant;
+
   // Initialized after symbolClass has been resolved.
   FunctionElement symbolConstructor;
 
@@ -916,6 +919,9 @@ abstract class Compiler implements DiagnosticListener {
     backend.initializeHelperClasses();
 
     dynamicClass.ensureResolved(this);
+
+    proxyConstant = constantHandler.compileVariable(
+        coreLibrary.find('proxy'), isConst: true);
   }
 
   Element _unnamedListConstructor;