Disallow `null` in spanFromSpannable.

pkg/compiler/lib/src/resolved_uri_translator.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.
 
 import 'common.dart';
 import 'elements/elements.dart' show LibraryElement;
 import 'util/emptyset.dart';
 
 /// API used by the library loader to translate internal SDK URIs into file
 /// system readable URIs.
@@ skipping 17 matching lines @@
   ///
   /// The [importingLibrary] holds the library importing [uri] or `null` if
   /// [uri] is loaded as the main library. The [importingLibrary] is used to
   /// grant access to internal libraries from platform libraries and patch
   /// libraries.
   ///
   /// If the [uri] is not accessible from [importingLibrary], this method is
   /// responsible for reporting errors.
   ///
   /// See [LibraryLoader] for terminology on URIs.
-  Uri translate(LibraryElement importingLibrary, Uri uri,
-      [Spannable spannable]);
+  Uri translate(LibraryElement importingLibrary, Uri uri, Spannable spannable);
 }
 
 /// A translator that forwards all methods to an internal
 /// [ResolvedUriTranslator].
 ///
 /// The translator to forward to may be set after the instance is constructed.
 /// This is useful for the compiler because some tasks that are instantiated at
 /// compiler construction time need a [ResolvedUriTranslator], but the data
 /// required to instantiate it cannot be obtained at construction time. So a
 /// [ForwardingResolvedUriTranslator] may be passed instead, and the translator
 /// to forward to can be set once the required data has been retrieved.
 class ForwardingResolvedUriTranslator implements ResolvedUriTranslator {
   ResolvedUriTranslator resolvedUriTranslator;
 
   /// Returns `true` if [resolvedUriTranslator] is not `null`.
   bool get isSet => resolvedUriTranslator != null;
 
   /// The opposite of [isSet].
   bool get isNotSet => resolvedUriTranslator == null;
 
   @override
   Uri translate(LibraryElement importingLibrary, Uri resolvedUri,
-          [Spannable spannable]) =>
+          Spannable spannable) =>
       resolvedUriTranslator.translate(importingLibrary, resolvedUri, spannable);
 
   @override
   Set<Uri> get disallowedLibraryUris =>
       resolvedUriTranslator?.disallowedLibraryUris ??
       const ImmutableEmptySet<Uri>();
 
   @override
   bool get mockableLibraryUsed => resolvedUriTranslator.mockableLibraryUsed;
 
   @override
   Map<String, Uri> get sdkLibraries => resolvedUriTranslator.sdkLibraries;
 }
 
 class _ResolvedUriTranslator implements ResolvedUriTranslator {
   final Map<String, Uri> _sdkLibraries;
   final DiagnosticReporter _reporter;
 
   Set<Uri> disallowedLibraryUris = new Set<Uri>();
   bool mockableLibraryUsed = false;
 
   _ResolvedUriTranslator(this._sdkLibraries, this._reporter);
 
   Map<String, Uri> get sdkLibraries => _sdkLibraries;
 
   @override
-  Uri translate(LibraryElement importingLibrary, Uri uri,
-      [Spannable spannable]) {
+  Uri translate(LibraryElement importingLibrary, Uri uri, Spannable spannable) {
     if (uri.scheme == 'dart') {
       return translateDartUri(importingLibrary, uri, spannable);
     }
     return uri;
   }
 
   /// Translates "resolvedUri" with scheme "dart" to a [uri] resolved relative
   /// to `options.platformConfigUri` according to the information in the file at
   /// `options.platformConfigUri`.
   ///
@@ skipping 51 matching lines @@
   }
 
   void registerDisallowedLibraryUse(Uri uri) {
     disallowedLibraryUris.add(uri);
   }
 
   Uri lookupLibraryUri(String libraryName) {
     return _sdkLibraries[libraryName];
   }
 }