fixes #197, don't generate casts that always pass

lib/src/codegen/js_codegen.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_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 235 matching lines @@
     return 'Symbol';
   }
 
   bool isPublic(String name) => !name.startsWith('_');
 
   @override
   visitAsExpression(AsExpression node) {
     var from = getStaticType(node.expression);
     var to = node.type.type;
 
+    // Skip the cast if it's not needed.
+    if (rules.isSubTypeOf(from, to)) return _visit(node.expression);
+
     // All Dart number types map to a JS double.
     if (rules.isNumType(from) &&
         (rules.isIntType(to) || rules.isDoubleType(to))) {
       // TODO(jmesserly): a lot of these checks are meaningless, as people use
       // `num` to mean "any kind of number" rather than "could be null".
       // The core libraries especially suffer from this problem, with many of
       // the `num` methods returning `num`.
       if (!rules.isNonNullableType(from) && rules.isNonNullableType(to)) {
         // Converting from a nullable number to a non-nullable number
         // only requires a null check.
@@ skipping 2713 matching lines @@
 
 /// A special kind of element created by the compiler, signifying a temporary
 /// variable. These objects use instance equality, and should be shared
 /// everywhere in the tree where they are treated as the same variable.
 class TemporaryVariableElement extends LocalVariableElementImpl {
   TemporaryVariableElement.forNode(Identifier name) : super.forNode(name);
 
   int get hashCode => identityHashCode(this);
   bool operator ==(Object other) => identical(this, other);
 }