Enable strong mode in DDC, fix all warnings/errors

lib/src/compiler/module_builder.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.
 
 import 'package:path/path.dart' as path;
 
 import '../js_ast/js_ast.dart';
 import 'js_names.dart';
 
 /// Base class for compiling ES6 modules into various ES5 module patterns.
@@ skipping 90 matching lines @@
           "Imports", new ArrayInitializer(importNames, multiline: true)),
       resultModule
     ]);
     return new Program(<ModuleItem>[moduleDef]);
   }
 }
 
 /// Generates node modules.
 class NodeModuleBuilder extends _ModuleBuilder {
   Program build(Program module) {
-    var importStatements = [];
+    var importStatements = <Statement>[];
 
     // Collect imports/exports/statements.
     visitProgram(module);
 
     for (var import in imports) {
       // TODO(jmesserly): we could use destructuring once Atom supports it.
       var moduleVar =
           new TemporaryId(pathToJSIdentifier(import.from.valueWithoutQuotes));
       importStatements
           .add(js.statement('const # = require(#);', [moduleVar, import.from]));
@@ skipping 56 matching lines @@
   // Ensure the identifier first character is not numeric and that the whole
   // identifier is not a keyword.
   if (result.startsWith(new RegExp('[0-9]')) || invalidVariableName(result)) {
     return '\$$result';
   }
   return result;
 }
 
 // Invalid characters for identifiers, which would need to be escaped.
 final _invalidCharInIdentifier = new RegExp(r'[^A-Za-z_$0-9]');