Support general expressions as keys in literal maps.

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

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 9591e18d0b00bd6d6bddc063a50d3ac362a11f4c..d6513dc35192e85ede4aa7e0850583120d15d2f9 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -1141,14 +1141,17 @@ class JavaScriptBackend extends Backend {
   }
 
   void registerConstantMap(TreeElements elements) {
-    Element e = compiler.findHelper(const SourceString('ConstantMap'));
-    if (e != null) {
-      enqueueClass(compiler.enqueuer.resolution, e, elements);
-    }
-    e = compiler.findHelper(const SourceString('ConstantProtoMap'));
-    if (e != null) {
-      enqueueClass(compiler.enqueuer.resolution, e, elements);
+    void enqueue(SourceString name) {
+      Element e = compiler.findHelper(name);
+      if (e != null) {
+        enqueueClass(compiler.enqueuer.resolution, e, elements);
+      }
     }
+
+    enqueue(MapConstant.DART_CLASS);
+    enqueue(MapConstant.DART_PROTO_CLASS);
+    enqueue(MapConstant.DART_STRING_CLASS);
+    enqueue(MapConstant.DART_GENERAL_CLASS);
   }
 
   void codegen(CodegenWorkItem work) {