Produce error when duplicate field initializers are found

lib/compiler/implementation/resolver.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.
 
 interface TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   Type getType(TypeAnnotation annotation);
 }
 
@@ skipping 411 matching lines @@
     visitor.warning(node, kind, arguments);
   }
 
   bool isFieldInitializer(SendSet node) {
     if (node.selector.asIdentifier() == null) return false;
     if (node.receiver == null) return true;
     if (node.receiver.asIdentifier() == null) return false;
     return node.receiver.asIdentifier().isThis();
   }
 
+  void checkForDuplicateInitializers(SourceString name, Node init) {
+    if (initialized.containsKey(name)) {
+      error(init, MessageKind.DUPLICATE_INITIALIZER, [name]);
+      warning(initialized[name], MessageKind.ALREADY_INITIALIZED, [name]);
+    }
+    initialized[name] = init;
+  }
+
   void resolveFieldInitializer(FunctionElement constructor, SendSet init) {
     // init is of the form [this.]field = value.
     final Node selector = init.selector;
     final SourceString name = selector.asIdentifier().source;
     // Lookup target field.
     Element target;
     if (isFieldInitializer(init)) {
       final ClassElement classElement = constructor.getEnclosingClass();
       target = classElement.lookupLocalMember(name);
       if (target === null) {
         error(selector, MessageKind.CANNOT_RESOLVE, [name]);
       } else if (target.kind != ElementKind.FIELD) {
         error(selector, MessageKind.NOT_A_FIELD, [name]);
       } else if (!target.isInstanceMember()) {
         error(selector, MessageKind.INIT_STATIC_FIELD, [name]);
       }
     } else {
       error(init, MessageKind.INVALID_RECEIVER_IN_INITIALIZER);
     }
     visitor.useElement(init, target);
-    // Check for duplicate initializers.
-    if (initialized.containsKey(name)) {
-      error(init, MessageKind.DUPLICATE_INITIALIZER, [name]);
-      warning(initialized[name], MessageKind.ALREADY_INITIALIZED, [name]);
-    }
-    initialized[name] = init;
+    checkForDuplicateInitializers(name, init);
     // Resolve initializing value.
     visitor.visitInStaticContext(init.arguments.head);
   }
 
   Element resolveSuperOrThisForSend(FunctionElement constructor,
                                     FunctionExpression functionNode,
                                     Send call) {
     // Resolve the selector and the arguments.
     ResolverTask resolver = visitor.compiler.resolver;
     visitor.inStaticContext(() {
@@ skipping 77 matching lines @@
     }
     return null;
   }
 
   /**
    * Resolve all initializers of this constructor. In the case of a redirecting
    * constructor, the resolved constructor's function element is returned.
    */
   FunctionElement resolveInitializers(FunctionElement constructor,
                                       FunctionExpression functionNode) {
+    // Keep track of all "this.param" parameters specified for constructor so
+    // that we can ensure that fields are initialized only once.
+    FunctionSignature functionParameters =
+        constructor.computeSignature(visitor.compiler);
+    functionParameters.forEachParameter((Element element) {
+      if (element.kind === ElementKind.FIELD_PARAMETER) {
+        checkForDuplicateInitializers(element.name,
+                                      element.parseNode(visitor.compiler));
+      }
+    });
+
     if (functionNode.initializers === null) {
       initializers = const EmptyLink<Node>();
     } else {
       initializers = functionNode.initializers.nodes;
     }
     FunctionElement result;
     bool resolvedSuper = false;
     for (Link<Node> link = initializers;
          !link.isEmpty();
          link = link.tail) {
@@ skipping 1893 matching lines @@
   TopScope(LibraryElement library) : super(null, library);
   Element lookup(SourceString name) {
     return library.find(name);
   }
 
   Element add(Element newElement) {
     throw "Cannot add an element in the top scope";
   }
   String toString() => '$element';
 }