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

dart/lib/compiler/implementation/apiimpl.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.
 
 #library('leg_apiimpl');
 
 #import('dart:uri');
 
 #import('../compiler.dart', prefix: 'api');
 #import('leg.dart', prefix: 'leg');
@@ skipping 19 matching lines @@
   elements.LibraryElement scanBuiltinLibrary(String path) {
     Uri uri = libraryRoot.resolve(DART2JS_LIBRARY_MAP[path]);
     elements.LibraryElement library = scanner.loadLibrary(uri, null);
     return library;
   }
 
   void log(message) {
     handler(null, null, null, message, false);
   }
 
-  leg.Script readScript(Uri uri, [tree.ScriptTag node]) {
+  leg.Script readScript(Uri uri, [tree.Node node]) {
     if (uri.scheme == 'dart') {
       uri = translateDartUri(uri, node);
     } else if (uri.scheme == 'package') {
       uri = translatePackageUri(uri, node);
     }
     String text = "";
     try {
       // TODO(ahe): We expect the future to be complete and call value
       // directly. In effect, we don't support truly asynchronous API.
       text = provider(uri).value;
     } catch (var exception) {
-      cancel("${uri}: $exception", node: node);
+      if (node !== null) {
+        cancel("$exception", node: node);
+      } else {
+        reportDiagnostic(const leg.SourceSpan(null, null, null),
+                         "$exception", true);
+        throw new leg.CompilerCancelledException("$exception");
+      }
     }
     SourceFile sourceFile = new SourceFile(uri.toString(), text);
     return new leg.Script(uri, sourceFile);
   }
 
-  translateDartUri(Uri uri, tree.ScriptTag node) {
+  translateDartUri(Uri uri, tree.Node node) {
     String path = DART2JS_LIBRARY_MAP[uri.path];
     if (path === null || uri.path.startsWith('_')) {
       reportError(node, 'library not found ${uri}');
       return null;
     }
     if (uri.path == 'dom_deprecated'
         || uri.path == 'html' || uri.path == 'io') {
       // TODO(ahe): Get rid of mockableLibraryUsed when test.dart
       // supports this use case better.
       mockableLibraryUsed = true;
     }
     return libraryRoot.resolve(path);
   }
 
-  translatePackageUri(Uri uri, tree.ScriptTag node) =>
-    packageRoot.resolve(uri.path);
+  translatePackageUri(Uri uri, tree.Node node) => packageRoot.resolve(uri.path);
 
   bool run(Uri uri) {
     try {
       packageRoot = uri.resolve('packages/');
       bool success = super.run(uri);
       for (final task in tasks) {
         log('${task.name} took ${task.timing}msec');
       }
       return success;
     } finally {
       packageRoot = null;
     }
   }
 
   void reportDiagnostic(leg.SourceSpan span, String message, bool fatal) {
     if (span === null) {
       handler(null, null, null, message, fatal);
     } else {
       handler(span.uri, span.begin, span.end, message, fatal);
     }
   }
 
   bool get isMockCompilation() {
     return mockableLibraryUsed
       && (options.indexOf('--allow-mock-compilation') !== -1);
   }
 }