Generate less code when importing dart:mirrors.

dart/sdk/lib/_internal/lib/js_mirrors.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 dart._js_mirrors;
 
 import 'dart:async';
 import 'dart:mirrors';
 
 import 'dart:_foreign_helper' show JS, JS_CURRENT_ISOLATE;
 import 'dart:_collection-dev' as _symbol_dev;
 import 'dart:_js_helper' show
     BoundClosure,
     Closure,
     JSInvocationMirror,
     Null,
     Primitives,
     RuntimeError,
     createUnmangledInvocationMirror;
 import 'dart:_interceptors' show Interceptor;
 import 'dart:_js_names';
 
-/// No-op method that is called to inform the compiler that
-/// tree-shaking needs to be disabled.
+/// No-op method that is called to inform the compiler that tree-shaking needs
+/// to be disabled.
 disableTreeShaking() => preserveNames();
 
+/// No-op method that is called to inform the compiler that metadata must be
+/// preserved at runtime.
+preserveMetadata() {}
+
 String getName(Symbol symbol) {
   preserveNames();
   return n(symbol);
 }
 
 class JsMirrorSystem implements MirrorSystem {
   TypeMirror get dynamicType => _dynamicType;
   TypeMirror get voidType => _voidType;
 
   final static TypeMirror _dynamicType =
@@ skipping 141 matching lines @@
     addToResult(Symbol key, Mirror value) {
       result[key] = value;
     }
     functions.forEach(addToResult);
     getters.forEach(addToResult);
     setters.forEach(addToResult);
     variables.forEach(addToResult);
     return result;
   }
 
-  List<InstanceMirror> get metadata => _metadata.map(reflect).toList();
+  List<InstanceMirror> get metadata {
+    preserveMetadata();
+    return _metadata.map(reflect).toList();
+  }
 }
 
 String n(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);
 
 Symbol s(String name) {
   if (name == null) return null;
   return new _symbol_dev.Symbol.unvalidated(name);
 }
 
 final JsMirrorSystem currentJsMirrorSystem = new JsMirrorSystem();
@@ skipping 378 matching lines @@
   String get _prettyName => 'VariableMirror';
 
   // TODO(ahe): Improve this information and test it.
   TypeMirror get type => JsMirrorSystem._dynamicType;
 
   DeclarationMirror get owner => _owner;
 
   Symbol get qualifiedName => computeQualifiedName(owner, simpleName);
 
   List<InstanceMirror> get metadata {
+    preserveMetadata();
     if (_metadata == null) {
       _metadata = (_metadataFunction == null)
           ? const [] : JS('', '#()', _metadataFunction);
     }
     return _metadata.map(reflect).toList();
   }
 
   static int fieldCode(int code) {
     if (code >= 60 && code <= 64) return code - 59;
     if (code >= 123 && code <= 126) return code - 117;
@@ skipping 114 matching lines @@
 }
 
 Symbol computeQualifiedName(DeclarationMirror owner, Symbol simpleName) {
   if (owner == null) return simpleName;
   String ownerName = n(owner.qualifiedName);
   if (ownerName == '') return simpleName;
   return s('$ownerName.${n(simpleName)}');
 }
 
 List extractMetadata(victim) {
+  preserveMetadata();
   var metadataFunction = JS('', '#["@"]', victim);
   return (metadataFunction == null)
       ? const [] : JS('', '#()', metadataFunction);
 }