Rewrite how we handle synthesized constructors in the compiler. This was motivated by issue https:/…

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

 // Copyright (c) 2012, 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 js_backend;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 3406 matching lines @@
     return globalMetadataMap.putIfAbsent(string, () {
       globalMetadata.add(string);
       return globalMetadata.length - 1;
     });
   }
 
   jsAst.Fun extendWithMetadata(FunctionElement element, jsAst.Fun code) {
     if (!backend.retainMetadataOf(element)) return code;
     return compiler.withCurrentElement(element, () {
       List<int> metadata = <int>[];
-      FunctionSignature signature = element.functionSignature;
+      FunctionSignature signature = element.computeSignature(compiler);
       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)));
       });
@@ skipping 456 matching lines @@
 
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;