Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/ssa/variable_allocator.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.
 
 import '../common.dart';
-import '../compiler.dart' show Compiler;
 import '../js_backend/js_backend.dart';
 import 'nodes.dart';
 
 /**
  * The [LiveRange] class covers a range where an instruction is live.
  */
 class LiveRange {
   final int start;
   // [end] is not final because it can be updated due to loops.
   int end;
@@ skipping 689 matching lines @@
       if (!needsName(input)) {
         names.addAssignment(predecessor, input, phi);
       } else {
         names.addCopy(predecessor, input, phi);
       }
     }
 
     namer.allocateName(phi);
   }
 }