Support compilation of Hello World

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

 // Copyright (c) 2013, 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.ir_builder_task;
 
 import 'package:js_runtime/shared/embedded_names.dart'
     show JsBuiltin, JsGetName;
 
 import '../closure.dart' as closure;
@@ skipping 1663 matching lines @@
   ir.Primitive visitLiteralString(ast.LiteralString node) {
     assert(irBuilder.isOpen);
     return irBuilder.buildDartStringConstant(node.dartString);
   }
 
   ConstantValue getConstantForNode(ast.Node node) {
     return irBuilder.state.constants.getConstantValueForNode(node, elements);
   }
 
   ConstantValue getConstantForVariable(VariableElement element) {
-    return irBuilder.state.constants.getConstantValue(element.constant);
+    ConstantExpression constant = element.constant;
+    if (constant != null) {
+      return irBuilder.state.constants.getConstantValue(constant);
+    }
+    return null;
   }
 
   ir.Primitive buildConstantExpression(
       ConstantExpression expression, SourceInformation sourceInformation) {
     return irBuilder.buildConstant(
         irBuilder.state.constants.getConstantValue(expression),
         sourceInformation: sourceInformation);
   }
 
   /// Returns the allocation site-specific type for a given allocation.
@@ skipping 2319 matching lines @@
       _backend.jsInteropAnalysis.hasAnonymousAnnotation(element.contextClass);
 
   String getJsInteropTargetPath(FunctionElement element) {
     return '${_backend.namer.fixedBackendPath(element)}.'
         '${_backend.nativeData.getFixedBackendName(element)}';
   }
 
   DartType get jsJavascriptObjectType =>
       _backend.helpers.jsJavaScriptObjectClass.thisType;
 }