Make dart2js accept constant null values in string interpolations.

pkg/compiler/lib/src/compile_time_constants.dart

 // Copyright (c) 2012, 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.compile_time_constant_evaluator;
 
 import 'common.dart';
 import 'common/resolution.dart' show
     Resolution;
 import 'common/tasks.dart' show
@@ skipping 490 matching lines @@
     StringConstantValue initialStringValue = initialString.value;
     DartString accumulator = initialStringValue.primitiveValue;
     for (StringInterpolationPart part in node.parts) {
       AstConstant subexpression = evaluate(part.expression);
       if (subexpression == null) {
         return null;
       }
       subexpressions.add(subexpression.expression);
       ConstantValue expression = subexpression.value;
       DartString expressionString;
-      if (expression.isNum || expression.isBool) {
+      if (expression.isNum || expression.isBool || expression.isNull) {
         PrimitiveConstantValue primitive = expression;
         expressionString =
             new DartString.literal(primitive.primitiveValue.toString());
       } else if (expression.isString) {
         PrimitiveConstantValue primitive = expression;
         expressionString = primitive.primitiveValue;
       } else {
         // TODO(johnniwinther): Specialize message to indicated that the problem
         // is not constness but the types of the const expressions.
         return signalNotCompileTimeConstant(part.expression);
@@ skipping 750 matching lines @@
 class _CompilerEnvironment implements Environment {
   final Compiler compiler;
 
   _CompilerEnvironment(this.compiler);
 
   @override
   String readFromEnvironment(String name) {
     return compiler.fromEnvironment(name);
   }
 }