Return null from stringValue and call slowToString() instead of toString().

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(() {
       if (compilationUnit.kind === ElementKind.LIBRARY) {
         compiler.log("scanning library ${compilationUnit.script.name}");
       }
       scanElements(compilationUnit);
       if (compilationUnit.kind === ElementKind.LIBRARY) {
         processScriptTags(compilationUnit);
       }
       native.processNativeClasses(compiler, compilationUnit);
     });
   }
 
   void processScriptTags(LibraryElement library) {
     LinkBuilder<ScriptTag> imports = new LinkBuilder<ScriptTag>();
     Uri cwd = new Uri(scheme: 'file', path: compiler.currentDirectory);
     Uri base = cwd.resolve(library.script.name.toString());
     for (ScriptTag tag in library.tags.reverse()) {
       SourceString argument = tag.argument.value.copyWithoutQuotes(1, 1);
-      Uri resolved = base.resolve(argument.toString());
+      Uri resolved = base.resolve(argument.slowToString());
       if (tag.isImport()) {
         // It is not safe to import other libraries at this point as
         // another library could then observe the current library
         // before it fully declares all the members that are sourced
         // in.
         imports.addLast(tag);
       } else if (tag.isLibrary()) {
         if (library.libraryTag !== null) {
           compiler.cancel("duplicated library declaration", node: tag);
         } else {
           library.libraryTag = tag;
         }
       } else if (tag.isSource()) {
         Script script = compiler.readScript(resolved, tag);
         CompilationUnitElement unit =
           new CompilationUnitElement(script, library);
         compiler.withCurrentElement(unit, () => scan(unit));
       } else {
         compiler.cancel("illegal script tag: ${tag.tag}", node: tag);
       }
     }
     // TODO(ahe): During Compiler.scanBuiltinLibraries,
     // compiler.coreLibrary is null. Clean this up when there is a
     // 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.toString());
+      Uri resolved = base.resolve(argument.slowToString());
       if (resolved.toString() == "dart:core") {
         implicitlyImportCoreLibrary = false;
       }
       importLibrary(library, loadLibrary(resolved, tag), tag.prefix);
     }
     if (implicitlyImportCoreLibrary) {
       importLibrary(library, compiler.coreLibrary, null);
     }
   }
 
@@ skipping 50 matching lines @@
   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);
     });
   }
 }