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

dart/lib/compiler/implementation/scanner/scanner_task.dart

Index: dart/lib/compiler/implementation/scanner/scanner_task.dart
diff --git a/dart/lib/compiler/implementation/scanner/scanner_task.dart b/dart/lib/compiler/implementation/scanner/scanner_task.dart
index 19a782e778a28104d3640ef7d6464710d812f24d..483704734d9b4b43d490db320038dcccda05fc67 100644
--- a/dart/lib/compiler/implementation/scanner/scanner_task.dart
+++ b/dart/lib/compiler/implementation/scanner/scanner_task.dart
@@ -57,7 +57,7 @@ class ScannerTask extends CompilerTask {
         }
       } 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));
@@ -80,7 +80,7 @@ class ScannerTask extends CompilerTask {
       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);
@@ -113,7 +113,7 @@ class ScannerTask extends CompilerTask {
     }
   }
 
-  LibraryElement loadLibrary(Uri uri, ScriptTag node) {
+  LibraryElement loadLibrary(Uri uri, Node node) {
     bool newLibrary = false;
     LibraryElement library =
       compiler.universe.libraries.putIfAbsent(uri.toString(), () {
@@ -134,7 +134,7 @@ class ScannerTask extends CompilerTask {
                      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}');
       });
     }