some fixes for --strong warnings

lib/src/codegen/js_metalet.dart

 // Copyright (c) 2015, 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 dev_compiler.src.codegen.js_metalet;
 
 // TODO(jmesserly): import from its own package
 import 'package:dev_compiler/src/js/js_ast.dart';
 import 'package:dev_compiler/src/js/precedence.dart';
 
@@ skipping 112 matching lines @@
   accept(NodeVisitor visitor) => toExpression().accept(visitor);
 
   void visitChildren(NodeVisitor visitor) {
     toExpression().visitChildren(visitor);
   }
 
   /// This generates as either a comma expression or a call.
   int get precedenceLevel => variables.isEmpty ? EXPRESSION : CALL;
 
   Block _finishStatement(List<Statement> statements) {
-    var params = [];
-    var values = [];
+    var params = <TemporaryId>[];
+    var values = <Expression>[];
     var block = _build(params, values, new Block(statements));
     if (params.isEmpty) return block;
 
     var vars = [];
     for (int i = 0; i < params.length; i++) {
       vars.add(new VariableInitialization(params[i], values[i]));
     }
 
     return new Block(<Statement>[
       new VariableDeclarationList('let', vars).toStatement(),
@@ skipping 96 matching lines @@
   bool found = false;
   _IdentFinder(this.name);
 
   visitIdentifier(Identifier node) {
     if (node.name == name) found = true;
   }
   visitNode(Node node) {
     if (!found) super.visitNode(node);
   }
 }