Repeated prefixes and parser fixes.

dart/frog/leg/scanner/scanner_task.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.
 
 class ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name() => 'Scanner';
 
   void scan(CompilationUnitElement compilationUnit) {
     measure(() {
@@ skipping 40 matching lines @@
     // better way to access "dart:core".
     bool implicitlyImportCoreLibrary = compiler.coreLibrary !== null;
     for (ScriptTag tag in imports.toLink()) {
       // Now that we have processed all the source tags, it is safe to
       // start loading other libraries.
       SourceString argument = tag.argument.value.copyWithoutQuotes(1, 1);
       Uri resolved = base.resolve(argument.slowToString());
       if (resolved.toString() == "dart:core") {
         implicitlyImportCoreLibrary = false;
       }
-      importLibrary(library, loadLibrary(resolved, tag), tag.prefix);
+      importLibrary(library, loadLibrary(resolved, tag), tag);
     }
     if (implicitlyImportCoreLibrary) {
       importLibrary(library, compiler.coreLibrary, null);
     }
   }
 
   void scanElements(CompilationUnitElement compilationUnit) {
     Script script = compilationUnit.script;
     Token tokens;
     try {
@@ skipping 22 matching lines @@
           native.maybeEnableNative(compiler, element, uri);
           return element;
         });
     if (newLibrary) {
       compiler.withCurrentElement(library, () => scan(library));
     }
     return library;
   }
 
   void importLibrary(LibraryElement library, LibraryElement imported,
-                     LiteralString prefix) {
-    if (prefix !== null) {
-      library.define(new PrefixElement(prefix, imported, library), compiler);
+                     ScriptTag tag) {
+    if (tag !== null && tag.prefix !== null) {
+      SourceString prefix = tag.prefix.dartString.source;
+      Element e = library.find(prefix);
+      if (e === null) {
+        e = new PrefixElement(prefix, library);
+        library.define(e, compiler);
+      }
+      if (e.kind !== ElementKind.PREFIX) {
+        compiler.withCurrentElement(e, () {
+          compiler.reportWarning(new Identifier(e.position()),
+                                 'duplicated definition');
+        });
+        compiler.reportError(tag.prefix, 'duplicate defintion');
+      }
+      imported.forEachExport((Element element) {
+        Element existing = e.imported.putIfAbsent(element.name, () => element);
+        if (existing !== element) {
+          compiler.withCurrentElement(existing, () {
+            compiler.reportWarning(new Identifier(existing.position()),
+                                   'duplicated import');
+          });
+          compiler.withCurrentElement(element, () {
+            compiler.reportError(new Identifier(element.position()),
+                                 'duplicated import');
+          });
+        }
+      });
     } else {
       imported.forEachExport((Element element) {
         compiler.withCurrentElement(element, () {
           library.define(element, compiler);
         });
       });
     }
   }
 }
 
 class DietParserTask extends CompilerTask {
   DietParserTask(Compiler compiler) : super(compiler);
   final String name = 'Diet Parser';
 
   dietParse(CompilationUnitElement compilationUnit, Token tokens) {
     measure(() {
       ElementListener listener = new ElementListener(compiler, compilationUnit);
       PartialParser parser = new PartialParser(listener);
       parser.parseUnit(tokens);
     });
   }
 }