Don't report an internal error if attempting to compile a file that doesn't exist.

dart/lib/compiler/implementation/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 39 matching lines @@
         imports.addLast(tag);
       } else if (tag.isLibrary()) {
         tagState = checkTag(TagState.LIBRARY, tag);
         if (library.libraryTag !== null) {
           compiler.cancel("duplicated library declaration", node: tag);
         } else {
           library.libraryTag = tag;
         }
       } else if (tag.isSource()) {
         tagState = checkTag(TagState.SOURCE, tag);
-        Script script = compiler.readScript(resolved, tag);
+        Script script = compiler.readScript(resolved, argument);
         CompilationUnitElement unit =
             new CompilationUnitElement(script, library);
         compiler.withCurrentElement(unit, () => scan(unit));
       } else if (tag.isResource()) {
         tagState = checkTag(TagState.RESOURCE, tag);
         compiler.reportWarning(tag, 'ignoring resource tag');
       } 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.
       StringNode argument = tag.argument;
       Uri resolved = base.resolve(argument.dartString.slowToString());
       if (resolved.toString() == "dart:core") {
         implicitlyImportCoreLibrary = false;
       }
-      importLibrary(library, loadLibrary(resolved, tag), tag);
+      importLibrary(library, loadLibrary(resolved, argument), tag);
     }
     if (implicitlyImportCoreLibrary) {
       importLibrary(library, compiler.coreLibrary, null);
     }
   }
 
   void scanElements(CompilationUnitElement compilationUnit) {
     Script script = compilationUnit.script;
     Token tokens;
     try {
@@ skipping 12 matching lines @@
       }
       compiler.dietParser.dietParse(compilationUnit, tokens);
     } catch (CompilerCancelledException ex) {
       throw;
     } catch (var ex) {
       compiler.unhandledExceptionOnElement(compilationUnit);
       throw;
     }
   }
 
-  LibraryElement loadLibrary(Uri uri, ScriptTag node) {
+  LibraryElement loadLibrary(Uri uri, Node node) {
     bool newLibrary = false;
     LibraryElement library =
       compiler.universe.libraries.putIfAbsent(uri.toString(), () {
           newLibrary = true;
           Script script = compiler.readScript(uri, node);
           LibraryElement element = new LibraryElement(script);
           native.maybeEnableNative(compiler, element, uri);
           return element;
         });
     if (newLibrary) {
       compiler.withCurrentElement(library, () => scan(library));
       compiler.onLibraryLoaded(library, uri);
     }
     return library;
   }
 
   void importLibrary(LibraryElement library, LibraryElement imported,
                      ScriptTag tag) {
     if (!imported.hasLibraryName()) {
       compiler.withCurrentElement(library, () {
-        compiler.reportError(tag,
+        compiler.reportError(tag === null ? null : tag.argument,
                              'no #library tag found in ${imported.script.uri}');
       });
     }
     if (tag !== null && tag.prefix !== null) {
       SourceString prefix =
           new SourceString(tag.prefix.dartString.slowToString());
       Element e = library.find(prefix);
       if (e === null) {
         e = new PrefixElement(prefix, library, tag.getBeginToken());
         library.define(e, compiler);
@@ skipping 57 matching lines @@
   static final int RESOURCE = 4;
 
   /** Next state. */
   static final List<int> NEXT =
       const <int>[NO_TAG_SEEN,
                   IMPORT, // Only one library tag is allowed.
                   IMPORT,
                   SOURCE,
                   RESOURCE];
 }