Enable strong mode in DDC, fix all warnings/errors

lib/src/js_ast/printer.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.
 
 part of js_ast;
 
 
 class JavaScriptPrintingOptions {
   final bool shouldCompressOutput;
   final bool minifyLocalVariables;
@@ skipping 1388 matching lines @@
                    params = new Set<String>();
 
   void forEachVar(void fn(String v)) => vars.forEach(fn);
   void forEachParam(void fn(String p)) => params.forEach(fn);
 
   void collectVarsInFunction(FunctionExpression fun) {
     if (!nested) {
       nested = true;
       if (fun.params != null) {
         for (var param in fun.params) {
-          params.add(param.name);
+          // TODO(jmesserly): add ES6 support. Currently not needed because
+          // dart2js does not emit ES6 rest param or destructuring.
+          params.add((param as Identifier).name);
         }
       }
       fun.body.accept(this);
       nested = false;
     }
   }
 
   void visitFunctionDeclaration(FunctionDeclaration declaration) {
     // Note that we don't bother collecting the name of the function.
     collectVarsInFunction(declaration.function);
@@ skipping 277 matching lines @@
     declare(node.name);
     node.function.accept(this);
   }
 
   visitClassExpression(ClassExpression node) {
     declare(node.name);
     if (node.heritage != null) node.heritage.accept(this);
     for (Method element in node.methods) element.accept(this);
   }
 }