dart2js: Introduce HGetLength to replace strange use of HFieldGet

pkg/compiler/lib/src/ssa/ssa_tracer.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 ssa.tracer;
 
 import 'dart:async' show EventSink;
 
 import '../diagnostics/invariant.dart' show DEBUG_MODE;
 import '../js_backend/namer.dart' show Namer;
@@ skipping 220 matching lines @@
     if (node.isAssignOp) {
       String valueId = temporaryId(node.value);
       return 'ReadModifyWrite: $receiverId.$fieldName $op= $valueId';
     } else if (node.isPreOp) {
       return 'ReadModifyWrite: $op$receiverId.$fieldName';
     } else {
       return 'ReadModifyWrite: $receiverId.$fieldName$op';
     }
   }
 
+  String visitGetLength(HGetLength node) {
+    return 'GetLength: ${temporaryId(node.receiver)}';
+  }
+
   String visitLocalGet(HLocalGet node) {
     String localName = node.variable.name;
     return 'LocalGet: ${temporaryId(node.local)}.$localName';
   }
 
   String visitLocalSet(HLocalSet node) {
     String valueId = temporaryId(node.value);
     String localName = node.variable.name;
     return 'LocalSet: ${temporaryId(node.local)}.$localName to $valueId';
   }
@@ skipping 305 matching lines @@
   }
 
   String visitAwait(HAwait node) {
     return "Await: ${temporaryId(node.inputs[0])}";
   }
 
   String visitYield(HYield node) {
     return "Yield${node.hasStar ? "*" : ""}: ${temporaryId(node.inputs[0])}";
   }
 }