Use static final empty Modifier instead of null.

lib/compiler/implementation/ssa/builder.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.
 
 class Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 897 matching lines @@
 
   /**
    * Documentation wanted -- johnniwinther
    *
    * Invariant: [functionElement] must be an implementation element.
    */
   HGraph buildMethod(FunctionElement functionElement) {
     assert(invariant(functionElement, functionElement.isImplementation));
     FunctionExpression function = functionElement.parseNode(compiler);
     assert(function !== null);
-    assert(function.modifiers === null || !function.modifiers.isExternal());
+    assert(!function.modifiers.isExternal());
     assert(elements[function] !== null);
     openFunction(functionElement, function);
     function.body.accept(this);
     return closeFunction();
   }
 
   HGraph buildLazyInitializer(VariableElement variable) {
     HBasicBlock block = graph.addNewBlock();
     open(graph.entry);
     close(new HGoto()).addSuccessor(block);
@@ skipping 3449 matching lines @@
           new HSubGraphBlockInformation(elseBranch.graph));
 
     HBasicBlock conditionStartBlock = conditionBranch.block;
     conditionStartBlock.setBlockFlow(info, joinBlock);
     SubGraph conditionGraph = conditionBranch.graph;
     HIf branch = conditionGraph.end.last;
     assert(branch is HIf);
     branch.blockInformation = conditionStartBlock.blockFlow;
   }
 }