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 6d32001cb0a370a711e4981779f0a1706432fb77..b84a19b1ca3507611a0fa17656a5e55d2a92c1c8 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -1144,14 +1144,17 @@ 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) {