Disallow `null` in spanFromSpannable.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2016, 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 mock_compiler;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:compiler/compiler.dart' as api;
@@ skipping 272 matching lines @@
   static Future create(f(MockCompiler compiler)) {
     MockCompiler compiler = new MockCompiler.internal();
     return compiler.init().then((_) => f(compiler));
   }
 }
 
 class MockResolvedUriTranslator implements ResolvedUriTranslator {
   static final _emptySet = new Set();
 
   Uri translate(LibraryElement importingLibrary, Uri resolvedUri,
-          [Spannable spannable]) =>
+          Spannable spannable) =>
       resolvedUri;
   Set<Uri> get disallowedLibraryUris => _emptySet;
   bool get mockableLibraryUsed => false;
   Map<String, Uri> get sdkLibraries => const <String, Uri>{};
 }
 
 class CollectingTreeElements extends TreeElementMapping {
   final Map<Node, Element> map = new LinkedHashMap<Node, Element>();
 
   CollectingTreeElements(Element currentElement) : super(currentElement);
@@ skipping 86 matching lines @@
       trustTypeAnnotations: trustTypeAnnotations,
       enableTypeAssertions: enableTypeAssertions,
       enableUserAssertions: enableUserAssertions,
       expectedErrors: expectedErrors,
       expectedWarnings: expectedWarnings,
       outputProvider: outputProvider);
   compiler.registerSource(uri, code);
   compiler.diagnosticHandler = createHandler(compiler, code);
   return compiler;
 }