Support compilation of Hello World

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.constants.expressions;
 
 import '../common.dart';
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
@@ skipping 662 matching lines @@
   int _computeHashCode() => 13 * name.hashCode;
 
   @override
   bool _equals(SymbolConstantExpression other) {
     return name == other.name;
   }
 
   @override
   ConstantValue evaluate(
       Environment environment, ConstantSystem constantSystem) {
-    // TODO(johnniwinther): Implement this.
-    throw new UnsupportedError('SymbolConstantExpression.evaluate');
+    return constantSystem.createSymbol(environment.compiler, name);
   }
 
   @override
   DartType getKnownType(CoreTypes coreTypes) => coreTypes.symbolType;
 }
 
 /// Type literal.
 class TypeConstantExpression extends ConstantExpression {
   /// Either [DynamicType] or a raw [GenericType].
   final DartType type;
@@ skipping 1065 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }