Generate tear-off closures dynamically.

dart/sdk/lib/_internal/compiler/implementation/js_emitter/metadata_emitter.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.
 
 part of dart2js.js_emitter;
 
 class MetadataEmitter extends CodeEmitterHelper {
   /// A list of JS expressions that represent metadata, parameter names and
   /// type, and return types.
   final List<String> globalMetadata = [];
@@ skipping 24 matching lines @@
             metadata.add(task.constantReference(value));
           }
         }
       }
       if (metadata.isEmpty) return null;
       return js.fun(
           [], [js.return_(new jsAst.ArrayInitializer.from(metadata))]);
     });
   }
 
-  jsAst.Node reifyDefaultArguments(FunctionElement function) {
+  List<int> reifyDefaultArguments(FunctionElement function) {
     FunctionSignature signature = function.computeSignature(compiler);
-    if (signature.optionalParameterCount == 0) return null;
+    if (signature.optionalParameterCount == 0) return const [];
     List<int> defaultValues = <int>[];
     for (Element element in signature.orderedOptionalParameters) {
       Constant value =
           compiler.constantHandler.initialVariableValues[element];
       String stringRepresentation = (value == null)
           ? "null"
           : jsAst.prettyPrint(task.constantReference(value), compiler)
               .getText();
       defaultValues.add(addGlobalMetadata(stringRepresentation));
     }
-    return js.toExpression(defaultValues);
+    return defaultValues;
   }
 
   int reifyMetadata(MetadataAnnotation annotation) {
     Constant value = annotation.value;
     if (value == null) {
       compiler.reportInternalError(
           annotation, 'Internal error: value is null');
       return -1;
     }
     return addGlobalMetadata(
@@ skipping 37 matching lines @@
           buffer.write(metadata);
         }
       } else {
         throw 'Unexpected value in metadata: ${Error.safeToString(metadata)}';
       }
       buffer.write(',$n');
     }
     buffer.write('];$n');
   }
 
+  // TODO(ahe): Delete this method.
   jsAst.Fun extendWithMetadata(FunctionElement element, jsAst.Fun code) {
     if (!backend.retainMetadataOf(element)) return code;
+    List<int> metadata = computeMetadata(element);
+    code.body.statements.add(js.string(metadata.join(',')).toStatement());
+    return code;
+  }
+
+  List<int> computeMetadata(FunctionElement element) {
     return compiler.withCurrentElement(element, () {
+      if (!backend.retainMetadataOf(element)) return const <int>[];
       List<int> metadata = <int>[];
       FunctionSignature signature = element.functionSignature;
       if (element.isConstructor()) {
         metadata.add(reifyType(element.getEnclosingClass().thisType));
       } else {
         metadata.add(reifyType(signature.returnType));
       }
       signature.forEachParameter((Element parameter) {
         metadata
             ..add(reifyName(parameter.name))
             ..add(reifyType(parameter.computeType(compiler)));
       });
       Link link = element.metadata;
       // TODO(ahe): Why is metadata sometimes null?
       if (link != null) {
         for (; !link.isEmpty; link = link.tail) {
           metadata.add(reifyMetadata(link.head));
         }
       }
-      code.body.statements.add(js.string(metadata.join(',')).toStatement());
-      return code;
+      return metadata;
     });
   }
 }