Prepare for decoupling analyzer ASTs from element model.

pkg/dev_compiler/lib/src/compiler/source_map_printer.dart

Index: pkg/dev_compiler/lib/src/compiler/source_map_printer.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/source_map_printer.dart b/pkg/dev_compiler/lib/src/compiler/source_map_printer.dart
index 3a12972bec7030b8930519f847f32ce4c9151f8c..7556952f2d34b86b7ed7c82c2abb5e11a0e03d4b 100644
--- a/pkg/dev_compiler/lib/src/compiler/source_map_printer.dart
+++ b/pkg/dev_compiler/lib/src/compiler/source_map_printer.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/resolution_accessors.dart';
 import 'package:source_maps/source_maps.dart' hide Printer;
 import 'package:source_span/source_span.dart' show SourceLocation;
 
@@ -48,7 +49,7 @@ class SourceMapPrintingContext extends JS.SimpleJavaScriptPrintingContext {
       // parts.
       _currentTopLevelDeclaration = node;
       unit = node.getAncestor((n) => n is CompilationUnit);
-      sourcePath = unit.element.source.fullName;
+      sourcePath = elementForCompilationUnit(unit).source.fullName;
     }
 
     _mark(node.offset, _getIdentifier(node));