dart2js: Don't return `null` from void functions.

pkg/compiler/lib/src/elements/modelx.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 elements.modelx;
 
 import '../compiler.dart' show
     Compiler;
 import '../constants/constant_constructors.dart';
 import '../constants/constructors.dart';
@@ skipping 1282 matching lines @@
     }
     assert(invariant(this, constantCache != null,
         message: "Constant has not been computed for $this."));
     return constantCache;
   }
 
   void set constant(ConstantExpression value) {
     if (isPatch) {
       ConstantVariableMixin originVariable = origin;
       originVariable.constant = value;
-      return null;
+      return;
     }
     assert(invariant(
         this, constantCache == null || constantCache == value,
         message: "Constant has already been computed for $this. "
                  "Existing constant: "
                  "${constantCache != null ? constantCache.getText() : ''}, "
                  "New constant: ${value != null ? value.getText() : ''}."));
     constantCache = value;
   }
 }
@@ skipping 1653 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }