Disallow `null` in spanFromSpannable.

pkg/compiler/lib/src/compiler.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 dart2js.compiler_base;
 
 import 'dart:async' show EventSink, Future;
 
 import '../compiler_new.dart' as api;
 import 'cache_strategy.dart' show CacheStrategy;
@@ skipping 1774 matching lines @@
 
   SourceSpan spanFromHInstruction(HInstruction instruction) {
     Element element = _elementFromHInstruction(instruction);
     if (element == null) element = currentElement;
     SourceInformation position = instruction.sourceInformation;
     if (position == null) return spanFromElement(element);
     return position.sourceSpan;
   }
 
   SourceSpan spanFromSpannable(Spannable node) {
-    // TODO(johnniwinther): Disallow `node == null` ?
-    if (node == null) return null;
     if (node == CURRENT_ELEMENT_SPANNABLE) {
       node = currentElement;
     } else if (node == NO_LOCATION_SPANNABLE) {
       if (currentElement == null) return null;
       node = currentElement;
     }
     if (node is SourceSpan) {
       return node;
     } else if (node is Node) {
       return spanFromNode(node);
@@ skipping 443 matching lines @@
   _ElementScanner(this.scanner);
   void scanLibrary(LibraryElement library) => scanner.scanLibrary(library);
   void scanUnit(CompilationUnitElement unit) => scanner.scan(unit);
 }
 
 class _EmptyEnvironment implements Environment {
   const _EmptyEnvironment();
 
   String valueOf(String key) => null;
 }