Support compilation of Hello World

pkg/compiler/lib/src/js_backend/constant_system_javascript.dart

Index: pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
index add75637e5d282b2acfb571c26721b598796a740..f3d2b4c98e072432885e863e6fbecc5aaf57e05e 100644
--- a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
@@ -10,7 +10,7 @@ import '../constants/values.dart';
 import '../constant_system_dart.dart';
 import '../core_types.dart' show CoreTypes;
 import '../dart_types.dart';
-import '../elements/elements.dart' show ClassElement;
+import '../elements/elements.dart' show ClassElement, FieldElement;
 import '../tree/tree.dart' show DartString, LiteralDartString;
 import 'js_backend.dart';
 
@@ -345,6 +345,21 @@ class JavaScriptConstantSystem extends ConstantSystem {
     return new JavaScriptMapConstant(
         type, keysList, values, protoValue, onlyStringKeys);
   }
+
+  @override
+  ConstantValue createSymbol(Compiler compiler, String text) {
+    // TODO(johnniwinther): Create a backend agnostic value.
+    InterfaceType type = compiler.coreTypes.symbolType;
+    ConstantValue argument = createString(new DartString.literal(text));
+    Map<FieldElement, ConstantValue> fields = <FieldElement, ConstantValue>{};
+    JavaScriptBackend backend = compiler.backend;
+    backend.helpers.symbolImplementationClass.forEachInstanceField(
+        (ClassElement enclosingClass, FieldElement field) {
+      fields[field] = argument;
+    });
+    assert(fields.length == 1);
+    return new ConstructedConstantValue(type, fields);
+  }
 }
 
 class JavaScriptMapConstant extends MapConstantValue {