Generate less code when importing dart:mirrors.

dart/sdk/lib/_internal/lib/js_mirrors.dart

Index: dart/sdk/lib/_internal/lib/js_mirrors.dart
diff --git a/dart/sdk/lib/_internal/lib/js_mirrors.dart b/dart/sdk/lib/_internal/lib/js_mirrors.dart
index fd1dd30ad3060dafc01647dde627d9482d3d9630..dc58e3835cfcf565b726017f0cf619d1c8a44612 100644
--- a/dart/sdk/lib/_internal/lib/js_mirrors.dart
+++ b/dart/sdk/lib/_internal/lib/js_mirrors.dart
@@ -20,10 +20,14 @@ import 'dart:_js_helper' show
 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);
@@ -185,7 +189,10 @@ class JsLibraryMirror extends JsDeclarationMirror with JsObjectMirror
     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);
@@ -584,6 +591,7 @@ class JsVariableMirror extends JsDeclarationMirror implements VariableMirror {
   Symbol get qualifiedName => computeQualifiedName(owner, simpleName);
 
   List<InstanceMirror> get metadata {
+    preserveMetadata();
     if (_metadata == null) {
       _metadata = (_metadataFunction == null)
           ? const [] : JS('', '#()', _metadataFunction);
@@ -718,6 +726,7 @@ Symbol computeQualifiedName(DeclarationMirror owner, Symbol simpleName) {
 }
 
 List extractMetadata(victim) {
+  preserveMetadata();
   var metadataFunction = JS('', '#["@"]', victim);
   return (metadataFunction == null)
       ? const [] : JS('', '#()', metadataFunction);